There are scant sources for this topic : if you think Elizabeth is doing enough to be consistent with respect regarding the death of her father's heir's wife depends, I suppose, on the view you take of mourning in our era.

I tend towards the view that the habit of donning mourning among the middle and certainly the upper classes in our era was a continuum of the mourning habits which developed post 1660, with the growth of the "middling sort" who were keen to ape court manners and fashions, under the reign of Charles II and onwards after the Interregnum.

I consider that it was a serious and far reaching business: and that it is possible that Elizabeth was not doing enough at this point in the story, by merely wearing black ribbons to commemorate Mr Elliot's dead wife.

Let me explain what court mourning entailed: the Lord Chamberlain would dictate and announce the periods of mourning , when specified dark clothing would be worn, to commemorate the death of a person at court or of someone in the royal family and , in addition, he would also specify a period general mourning to affect those who were not active at court,but who considered themselves genteel enough to participate in mourning a public figure.

And not only was it then worn out of respect for the said prominent person, but this type of mourning was adopted by this set of people to mark deaths in their own family and circle of friends. If you can ever take a look at Anne Bucks wonderful book Dress in Eighteenth Century England (1979) she attempts to explain the levels of mourning worn by different classes of people during our era, and I highly recommend it.

For example,to look at Emma just for a moment, it is clear from Emma's reaction to the length of mourning that Jane Fairfax refers to regarding Mrs Churchill(deceased)l that there was some form of openly agreed mourning period for certain relatives,which was seen as appropriate, even if now we have no documentary evidence of it :

"You are very right; it has been thought of. And I will own to you, (I am sure it will be safe), that so far as our living with Mr. Churchill at Enscombe, it is settled. There must be three months, at least, of deep mourning; but when they are over, I imagine there will be nothing more to wait for."
Chapter 52

Emma does not demur at Jane's statment -therefore a 3 months deepest mourning period for a deceased aunt must have been agreed upon and thought correct in Emma's circle.

I have corresponded on this subject with the wonderful Penelope Byrde, the author of Jane Austen in Fashion and one time curator of the Museum of Costume in Bath. And if you don't mind I'll quote from one of her letters here as I think its interesting and relevant:

In essence,yes I agree with you that the mourning customs of the gneral middle class population did follow and evolve from court mourning. Orders for general public mourning were issued on the death of members of the royal family and these were generally observed by people of the middle class-as distinct from Court Mourning observed by a much narrower circle of the royal family and the Court. These instructions would have been disseminated through the press and by word of mouth-and because there were deaths in the royal family at fairly regular intervals( including more distant relatives) this would have been a recurrent event.

In addition, I believe there must have been a certian amount of custom and practise whihc was never acutally written down but which was passed on verbally and in letters. Surviving correspondence from the period (such as Mary Delaney or our JAne Austen) makes this very clear and advice was often sought from those who moved in fashionalbe society or who were considered to be experienced in these matters. Even today I think we would do this- for example, when wondering what to wear at a wedding we would ask friends or relations rather than rush to consult an etiquette book.

The general rules of mourning would be known but I am sure there were variations in interpretation. It is unlikely that there was a known relegation for very conceivable circumstances(e g death of a fiancé ) but individuals might adopt or adapt mourning dress and colours to suite their situation. Certainly it is true that mourning was worn throughout the 18th century/early 19th centuries by others than the court and from the surviving correspondence it would appear that the rules were quite detailed and generally known,even if they were not as obviously disseminated as the far reaching mourning period "tables" of the Victorian era

So....my theroy aobut Elizabeth's mournign ribbons is that perhaps she is simply not doing enough by wearing them. Mr Elliot was after all her father's heir despite their estrangement,and at this point in the story she ought to be wearing some more substantial outward recognition of that fact than mere ribbons. It would be in character for the self obsessed Elliots not to get it quite right. But that is just one theroy ;-)

But.....Another may be more helpful.

In the recently published Cambrige Edition of the works of JA, the editor of Persuasion, , Janet Todd, poses this threoy regarding the ribbons:

It is just possible that "she" is a printing error for "he" and that it is Mr Elliot at this time who is wearing the black ribbons (either added to his hat or to the knees of his breeches).
The Cambridge edition of 'Persuasion',page 338

These ribbons were also termed "weepers", and were worn during our era by men. The OED ( online edition) defines them as

A conventional badge of mourning.

and gives the following as sources for that statement:

1724 Lond. Gaz. No. 6255/2 All..being enjoyned to appear..in long black Cloaks, Cambrick Bands, Chamoy Shooes, Weepers, &c.

1746 H. WALPOLE Let. to Mann 1 Aug., I..was assisted by the sight of the Marquis of Lothian in weepers for his son who fell at Culloden.

1760-2 GOLDSM. Cit. W. xcvi, Our merry mourners clap bits of muslin on their sleeves, and these are called weepers.

1827 HOOD True Story 115 There comes some unexpected stroke And hangs a weeper on the cuff.

Look at this extract from a letter, written by a fairly desperate sounding and confused Lord Breadalbane, written in 1771 from Edinburgh to Lady Grey, his daughter in law, asking for detailed information as to what the present "regulations" about mourning were:

I am so out of the world that I don't know the regulations about mourning, which have gone through several alterations. Here they wear weepers for Uncles and Nephews ( which I think they don't in England) and even for cousins, and their mournings are very long, but as I think all ceremonial fashions shoud be regualted by those of the Court, I wish you would let me know how long I shoud wear weepers if I were in London,and how long black wax is used. I think weepers are worn half the whole mourning, black swords and Buckles without weepers during half the remainder and slight mourning of the last quarter of the time but I don't know how long the whole amounts to.

Page 63 Dress in Eighteenth Century England Anne Buck.

However, the only problem with this theory that I can see is that weepers were made from white linen, and therefore were not black:see this again form the OED:

A strip of white linen or muslin formerly worn on the cuff of a man's sleeve. Cf. F. pleureuse.

So there you are: many theories, and you can choose to accept one or none of them : that's as helpful as I can be I'm afraid ;-)