I really don't know when your parents and brother _是will 还是 would _ come back

I really don't know when your parents and brother _是will 还是 would _ come back
I really don't know when your parents and brother _是will 还是 would _ come back

I really don't know when your parents and brother _是will 还是 would _ come back
I really don't know when your parents and your brother will come back.
or. I really don't know when your parents and your brother came back.

will

用will。因为还没回来，所以用将来时

应该用will.
这个是现在时态。表示将来会发生什么。
would主要是虚拟语气或者敬语。如果要用would, 应该是I really didn't know when your parents and brother would come back.

用would可能会好些, 虚拟语气.