] ...I think she knows Henry well enough to know that he is not un interested. ;-) I also think that Eleanor is interested in Catherine for her own sake - here is a nice (such a useful word!) sweet, unaffected girl, pleasant company for someone who, I suspect, does not have much female company living in a houseful of men in the country.

I agree entirely with the above. But how about another twist on it?

I believe Henry and Eleanor, being such good people, have a genuine desire to be good influences to Catherine, regardless of wether Henry is looking for a wife or Eleanor for a friend. They see she has a good heart and has been brought up, as you said, "sweet" and "unaffected," and I might add, good-natured (but I won't say it more than once!!). But they also see that her relationship with the Thorpes is threatening to harm her sense of right.

I'm not suggesting that Catherine has started to do awful things; but little things like...
1) She didn't realize it was wrong to go by herself with JT in the carriage.
2) She is too caught up in the novels with Isabella, who probably wouldn't be able to actually discuss a book to save her life.
3) Her judgment is not refined; she cannot yet see through Isabella and John (though I applauded like crazy this week when she "could almost have accused Isabella of being
wanting in tenderness towards herself and her sorrows" and thought that "John Thorpe himself was quite disagreeable." YAY CATHERINE!!)

I can very much see them wanting to improve her, not like Emma tries to improve Harriet, but truly for Catherine's sake.

For example, in Ch. 10, Henry says, That gentleman would have put me out of patience, had he stayed with you half a minute longer. He has no business to withdraw the attention of my partner from me regarding JT. Although it is said in jest, it marks his dissatisfaction in the acquaintance--for any modest, unaffected girl I believe, but I think it is becoming especially so for Catherine :)

Of course, since they desire her friendship, it would also be better for them if Catherine had the same/similar sense of decorum. So it would be better all around.

] I don't think that Eleanor or Henry are seriously thinking of matrimony, but the possibility may have crossed their minds.... ;-)