I can relate to that. I hate it when people simply ignore me. When it's family it is a bit tough to handle. Otherwise, I just do the same.
I have been told by a friend that I am very good at ignoring people BUT I ignore people I don't like and I don't invite them to parties. I know a few people who invited me to rock the boat but they were not interested in me. In that case, I see if I benefit from the event and consider it as a exchange. If I don't benefit from it, I don't go. Don't go to your sister-in-laws next party. See her reaction. Force her to come to you and ask you. If she doesn't, forget her at least for a while.
If she invites you again, I would smile and tell her in a friendly but certain manner that you have another invitation and that anyway, she has so many guests, she won't even notice you are not here. But don't say that in a reproachful or complaining manner. Say it as if you were trying to comfort HER. "OK, next time, I will do my best to come. Promise! Kissy kissy. Nice dress ! Bye !".
I just noticed that a childhood friend of mine answers my e-mails only if she can display some knowledge. Otherwise she feels inferior because she is very complexed. Not knowing increases this feeling. So actually her "indifference" is a compliment to me !!! Crazy, no ?

It is hard to tell why people ignore you. It can be for many reasons : indifference, jealousy, momentary distraction, shyness but it is definitely not because you are not interesting. People have their own agenda. You have to find out on a case by case basis. Just ask directly if you feel they will give you an honest answer. Some people will not be honest, others will. And hey, if not, there are people out there who will show interest and if you start ignoring your former friends/sister-in-law, they may very well start to wake up.