Helping out your family in times of need can take a toll, but it's part of being a loving family member. However, just because someone is blood doesn't mean you're obligated to shirk off all boundaries in order to do them a favor. We all have our lines, and putting yourself in a position that will cause you to resent a loved one certainly isn't better than saying no.
That being said, sometimes you just have to grin and bear discomfort in order to help a loved one in need, so figuring out when to tap out or stay in the game is a toughie.
"AITA for refusing to clean my pregnant sister's disgusting house? A few details first: I am a big man. My sister is a tiny pregnant woman. Her husband is a seafarer and currently at sea. They have a boatload of money."