Engagement: Unsupportive siblings?

Hi guys! Me and my Fiancé got engaged back in august and we are so excited to start planning!

The unfortunate part about our situation is that even though we both have awesome families, his siblings have proven to be EXTREMELY rude, unsupportive and vindictive for really no other reason besides they’re assholes. The most of the crap we get is from his sister and sister-in-law, but his sister is the worst. I realize not everyone is going to be overjoyed for us, and that’s okay but I’m just struggling with constantly dealing with nasty comments and little digs about our lives and the lack of positivity towards our future marriage. We’ve always gotten along like sisters and we’ve never had a problem, I honestly think his sister is jealous that her boyfriend hasn’t proposed to her yet and she’s projecting her unhappiness onto us, and I’m just having a hard time keeping the peace and ignoring it. Any advice? I’d understand if I’ve ever been unsupportive of anything in their lives but it’s always been the opposite, I’ve always been involved in every event and extremely happy for anything good they have going on, I’m so confused. It’s really making me worry about whether or not I want them as bridesmaids If they’re gonna be like that too... help!