Problem with boyfriend of 3.5 years younger sister?

Any sister would be jealous of her big brother's girlfriend, especially when its serious. She knows you guys are serious about each other so maybe she's trying to break up in her own way. That's why she says those hurtful words. Think about it. Were you and his sister even close to begin with? Even if you guys were just okay with each other, if you didn't have a close relationship with her then she'll feel like a stranger is just taking her brother away.