One of the best ways to keep others at bay is to pay for your own wedding. If you accept money from his family, then his mother and sister will have a right to have a say in how it is spent. You and your fiance need to sit down together and agree on what the two of you want - every detail, from number of guests to food. Then, move forward with your plan. When someone offers their ideas, smile and say you will discuss it with your fiance and let it go at that.
If things get too chaotic, you might hire a wedding coordinator or planner. They don't have to cost a fortune and a good planner will act as a buffer between you and the family. Plus, with her knowledge, she may actually save you money.