Can you talk to your mum and sister? I mean, if you could say "Look, I'm really happy about the whole wedding thing, and I'm definitely hoping to lose weight, but I don't find that it helps me to feel like I'm being continually pestered about it, so if you really want to help, could you maybe just give me some space to go for it on my own?" then would they take that on board?
I'm losing a bit of weight at the moment, though not on the same scale, and I know how I react to certain things is not helpful. Like, if I think "I can't have that", then my next thought will be "Well I'm damn well going to have it, no-one is going to stop me eating what I want!", which rather ruins the healthy eating plan.
But if I can catch that thought before it pops up and replace it with "I'm choosing not to eat that because losing weight is more important to me" then I feel positive about my choice to not have it, rather than negative, and that keeps me on track much better.
If you know that your mum and sister's comments and behaviour set you into a negative feeling about losing weight, then getting them to understand that and try to either be helpful or just keep quiet would maybe help.