I've helped out my mother in her business for years, on evenings and weekends. Overall, I haven't found it a good experience, mainly because of two issues.

Firstly, I really don't have the time to put her affairs in order (and keep them that way), only enough to put out fires. I do get paid, but it's really not a matter about money. As you say, it then begins to pollute the relationship with that person, since you can no longer spend time with them normally.

Secondly, it's quite painful to see bad decisions being made against your advice when it's someone you know and care about. It also makes the motivation to put out fires much lower, since you know there'll always be more on the way...

Having chatted to friends, I understand that the situation becomes much easier where both parties have some kind of help they can trade with each other, instead of most of the help going one way.