Fiancés sister hates me?

Well the title says it all. She’s one of those people who’s very sweet and kind upfront when you talk to her but her actions say otherwise. His other two brothers are so sweet and she is also but only upfront. There’s been many occasions where she was nice to me when I actually talk to her but she’s purposely left me out of things and un invited me to things with excuses and pushy to me when my fiancé isn’t around. first I thought maybe I just didn’t know her well enough or I didn’t know the whole story, and I know I was cold towards my brothers fiancé at first just because I was a protective sister but I never treated her like THAT. My fiancé and I don’t have issues, we don’t fight, we are very loving and supporting of each-other, I love him so much and honestly want the best for him and I know I have that in common with his sister so I don’t know why she feels this way towards me? I thought it was in my head until my fiancé told me he does think his sister has a very mean girl attitude towards me as well. I’ve tried to reach out to her, bring her foods and coffee when I stop by, I even have tried to set up things for us to do like going out for a girls night and getting our nails done or getting coffee and I’ve brought her food and stuff, I feel like I’ve made an effort but none of it has worked to get her to realize I don’t mean any harm and I’d even like a sister relationship with her someday. I’ve asked my fiancé if she has any ill will or reason to how she feels towards me and he says she’s just kind of being a brat and I shouldn’t take it personally because I didn’t do anything wrong. I’m just getting to the point where I don’t feel comfortable making an effort to someone who has clearly not been nice to me, but at the same time I know that’s his sister and they both are very close so if he’s going to be my husband it would be weird if the sister he’s so close with hates me for no reason and puts me in a weird position. It’s getting to the point where it’s awkward when I’m around them because they are so tight knits and close and I don’t feel included because I know she doesn’t like me.🤣 I’m not sure what to do in this position?

I want to be able to get to the point where I can be close with her but at the same time I know she’s purposely tried to hurt me and I don’t want to get walked over either.