A dog owner who travels frequently leaves her dog with her sister-in-law to watch him for her, never offering to pay her for it, saying that 'family doesn't charge each other.' When she tries to ask her to watch her cat for once, the other demands she be paid $50 a day. 

What's worse than a cheap person? A cheap person who's also a hypocrite. 

There's a big difference between asking your family members for a favor and taking advantage of their generosity and goodwill. We all need help sometimes, and it's okay to ask our close relatives to help us when we have issues we can't solve on our own. That's all good, and it's what family's for, but there needs to be some kind of boundary or reciprocity. We can't expect them to come running for us all the time without us being willing to help them when they need it. Relationships and family bonds are reciprocated, not one-sided. Otherwise, it's completely unfair for the person who does the helping. 

When it comes to a big favour, like babysitting our pets, it's important that we consult with them in advance. Taking care of an animal is a lot of work and a big responsibility, especially if it's someone else's animal. Their usual lifestyle might not agree with having a pet, and we have to respect that. 

In my case, I've had friends babysit my pets, and I've done the same with them, but we always agree on the terms beforehand. Each of us drops by the pet with their food and all the necessary things, and we never ask the other to take charge of things that aren't their responsibility. We buy all the expensive things in advance nd we leave them there. We've never demanded the other give us money for something we are both willing to do as a favor, and that's the difference between us and the sister-in-law who likes to have favors done for her but is not willing to reciprocate.