Actually the commas make a lot of sense to me. :-)

JA's sentences are always long and one has to always go back to refer to the actual topic to which the last part refers to.

"Her form, though not so correct as her sister's, in having the advantage of height, was more striking;"

If the last comma weren't there then it would have been one continuous thought;

"..in having the advantage of height was more striking;"

The fact that it has been separated by a comma indicates to me that this part does not refer to what was said before the comma.

I think I better get down to more constructive discussion about Brandon, Elinor Marianne etc. :-/