OP has been in a child-free relationship with his girlfriend for 8 years. She's introverted, has anxiety, and considers their home her safe space.
OP occasionally babysits his sister's three young kids (ages 7, 5, and 3) at their house because it's more convenient with all their stuff there, and OP's girlfriend prefers their home to be a quiet, safe space due to her anxiety.
OP's brother-in-law asked if he could drop the kids off with OP, but OP insisted on going to their place. He got upset, said the kids needed fresh air, and called OP selfish, refusing his offer to go to the park together.
OP is offering a service, and nobody has a right to demand it or access to his home.
They can ask, but demanding is not acceptable.
OP is not at fault for refusing to babysit at their home due to his girlfriend's preferences.
OP is not at fault in this situation.
It's unreasonable for OP's BIL to expect his children to visit OT's home when he is already doing him a favor.
OP's girlfriend has a right to feel safe and comfortable in her shared home.
OP's brother-in-law's behavior is inconsiderate.
OP's brother-in-law is extremely entitled, expecting free childcare from OP without considering his boundaries.
OP's family expects free babysitting on their terms.
If this arrangement doesn't work for the parents, they should find an alternative solution.
OP's decision to maintain his boundaries and not host his nieces and nephews at his home is reasonable and considerate of his girlfriend's preferences and their child-free lifestyle. He has the right to establish and maintain the rules and atmosphere within his own living space, especially when it aligns with his partner's wishes.
Additionally, his willingness to compromise by suggesting taking the kids to the park shows that he is not entirely dismissive of spending time with his family but is trying to find a middle ground. OP's decision to stick to his boundaries and find compromises that work for everyone involved is a fair and respectful approach to a challenging family situation.