Should a person wait to contact a lawyer while a person is still alive or wait till the person has past to contest a will or POA?

The most important questions that need to be considered, here, are first, whether or not your parents are still of reasonably sound mind, and second, whether the situation is such that your brother has effectively taken over your parents' lives, making them agree to what he wants and not what they want.

If you have concerns about the answer to either or both of these questions, you should probably talk to an elder care attorney.  There is a centuries-old legal principle that courts dislike, and under certain conditions will undo, transactions that lead to unjust enrichment, especially when a confidential relationship like family is exploited.  More recently, the law has recognized that senior citizens are at higher risk for financial abuse, as well.