Very wise of you not to tell her your feelings. If indeed it is her husband that is doing this, why put a wedge between them?
Perhaps, like others have said, she wants to identify with her new more wholesome family. Also, perhaps your brother-in-law put your whole family into one category, in his mind, and never thought of treating you differently. If the latter is the case then, like Aries said, work on the relationship of you and your husband towards your brother-in-law.
Please don’t just show up on her doorstep. The fastest way to destroy a relationship is to discuss it. You certainly don’t want to tail her, but you can use subtle remarks about how you wish you can meet more often.
As 565656 said, (perhaps it’s your sister) someone that received a lot of help may feel the need to back off. She might feel that you are mothering her. It would be an artfull task, but if you can, you should, by subtle statements, bring out the levelness and equality of the two of you. Try not to give advice, ask her opinion on many things, make small talk, and make sure you are not in any way imposing a view (like I am doing now).