I (F27) have a sister, Eni (24y), she married Tom (25yo) last year. Tom has a sister, Lily (23yo).
Lily is a bully. I have met her a handful of times and she is always making fun of Eni, who is a little fat, Lily's always telling that bad pig jokes and she even gave Eni a nickname, Pigni.
I have complained to Tom and Tom's mom that Lily's jokes weren't funny and she should stop, but they always said she was just joking. I told Eni that she should return the jokes to Lily, but she can never think of anything to say.
Last year I was invited to Tom's family's Christmas party, everything was fine until Lily said something like "Oh God Eni, you've already eaten so much, at this pace you'll stop being Pigni to become Whalni" and she laugh with the children at the party. I was furious and Eni and Tom gave a fake laugh and said that Eni was 4 months pregnant. When we were all congratulating them, Lily said "wow, at least now you can excuse yourself when people see you eating like a pig." I got furious again but Eni asked me to just let it go but I know how much that jokes hurt her. So I went back to having a talk with Tom and his mother about why Lily's teasing had to stop.
Well, this weekend was Eni's babyshower, we were having a good time until Lily told another joke about how "fat" Eni had gotten, I had enough so I laughed so loud to attract attention and said "Oh Lily, you're so funny, especially when you finish your jokes with that ugly pig laugh, sometimes I think people laugh more at your laugh than at your joke." Lily got super serious and she told me it wasn't funny, to which I said "come on, it's a joke" and imitated her laugh, the kids at the party laughed too and Lily ran away while she cried. Tom's mother scolded me for the joke and I asked her why Lily's jokes were funny but mine weren't, if they both had a pig joke in it, and her mother couldn't answer me. I told her that every time I heard Lily tell a joke about Eni, I would return the joke. Eni told me that I shouldn't have done that, but I could see her hold back her laughter along with the other guests, but I'm tired of Eni being walked over. They have told me that since I am older than Lily I should behave better.
So AITA for making fun of Lily?