Therapy is important, but you can make changes without it and stop self-sabotaging. You don’t have friends, but you do have a fiance. Keep failing to agree a wedding date, because you don’t have friends to invite, and you may also end up without a fiance. The guy wants to marry you. I doubt he cares how many friends you invite to the wedding. You don’t need a big wedding. You don’t need bridesmaids. Just have a wedding which makes you comfortable and get on with your relationship, even if it is just the two of you standing before a judge. It’s the marriage that is important, not the wedding.
You should talk to family about your issues. You should especially talk to your fiance. You are who you are. If they love you, they accept you as you are and will help you achieve your goal of developing strong friendships.
I don’t know whether your state of mind has caused you to over-react to your sister’s claiming your hobby friends or if she actively sabotaged you. Talk to her.