Can my brother and sister-in-law live in the master bedroom in my new house? Is there any feng shui problem?

Of course not. Brother or sister-in-law can, but brother and sister-in-law can't live together. It's even worse if you haven't lived there. If the new house is unoccupied, the house will be ownerless. When men and women live in the combination of yin and yang, the new house is the master, and the combination of yin and yang will bring vitality to everything, unless you want to give him your house, that is another matter. There is no feng shui problem, but it is more serious than feng shui.