My boyfriend's sister teases me, but my boyfriend gets mad. How can I tell her that the teasing annoys him?

A girl I'll call Tatiyana* always teases a boy I'll call Ilia* and I. Ilia* gets mad and he says stuff like, "AY! Shut it!", but he doesn't yell. Of course, Tatiyana* laughs it off like a crazy person and it makes me laugh. I tell him the teasing makes me really laugh, but Ilia* says it's annoying. I respect his opinion. Tatiyana* keeps doing it, and it gets more annoying for him. I laugh, but I secretly think it's a little annoying. They're siblings, so I know the situation very well. She constantly calls him "the next Ewan McGregor" (referring to the modern STAR WARS movies) and dubbed us "She+Him". She says stuff like "If it isn't She+Him" and "Can someone gouge my eyes out?" when we get affectionate. Tatiyana* and I get our girl time when he isn't around and we tell secrets, so I know she isn't calling "Mine!" with me or Ilia*. They both have Asperger's Syndrome, so getting their points across is kind of hard for them and I understand. With Tatiyana*, it's extremely hard and I am like an "in-law" to her because Ilia* is her brother and she sees me like that. I always figure that it's part of her ASD, so I try to help her with it, but she ends up teasing Ila* and I AGAIN. Ilia* ends up getting peeved and annoyed, yet he doesn't yell. When I tell Tatiyana* to stop it, she stops. When Ilia* tells her to stop, she refuses and he gets peeved. How can tell her that it annoys Ilia* when she teases him without getting her mad?