回複：請問: 丈夫把遺產留給侄子有效嗎?

It is hard to tell without knowing the jurisdiction and without seeing the actual will. Many jurisdictions have elected shares for spouses, provide homestead or provide for the pretermitted spouses. You need to consult an attorney on how to challenge a will. A will can be challenged in many ways. A will devising property unnaturally, such as omitting a spouse, can definitedly be challenged.