Brother-in-law's new house is occupied, can sister-in-law not go?

Of course. If you have something to do, you can't leave your body. Brothers and sisters can move to a new home, but they should say hello.

For brothers and sisters, moving to a new home is a happy thing. They are good brothers and sisters, and it is appropriate to celebrate. In some places, when this happens, you have to send out molecules (red envelopes). If you happen to have something important to deal with in this situation, or you are in a foreign country and can't come back, there is no way out. You can't leave your job. But if people don't go, they still have to call. Explain the situation and find a cold solution. I believe your brothers and sisters will forgive you, too.