Sister or Best Friend

That's a tough one. Do you actually know what went wrong? Because refusing to let your niece or nephew be involved either is pretty extreme. Unless there is a really good reason for your sister's reluctance, then I'd maybe start by talking to each of them, explaining how much it hurts you seeing two people who are so close to you being at odds with each other and ask if they can put the past behind them and start again.

If they won't, then it's up to you to decide who you most want by your side at your wedding. Who is going to be the most support to you? Because that's what you really need, someone who will be a real help to you, not just someone who is going to look good in a posh frock in the photos!

Whatever you do, don't give in to emotional blackmail. SOOOOO many people do the 'if you do that, then I won't come to your wedding/be your bridesmaid/go on your hen do', and it's just WRONG. (I've already had two people tell me that they won't come to my wedding unless I invite someone else they think should be there - in both cases, I've explained that I'd be really sorry if they don't feel able to come, but that my fiance and I have carefully discussed the guest list and are not able to alter it. In both cases, they've since accepted their invites!!! So I think often, the emotional blackmail is just an attempt to push you into doing what the other person wants)

If you were still close to your best friend then I'd say best friend for MOH. Your sister is family and will always love you and be there for you. I just had to make that decision and went with my best friend because I knew she would have more time to dedicate to me and the wedding and my needs and she would really take the job seriously ya know? Not that my sister wouldn't, but family tends to do more what they want than what you want sometimes.

Good luck!