She has no connections anymore. By herself, she's powerless. And when he's emperor, he'll have much more influence and controll over who she's talking to and communicating with. She'll just be his pitiful mother at that point. I doubt she'd want to do much anyway. Maybe even just be sent to a house where she can't meet anyone, but is on a beach or something.
Also, people are capable of changing and regretting their actions. Her entire goal was to make him emperor, and I think to absorb her homeland to be the ruler there as well. As long as that happens, I don't think she'll really care.
Overall, its definitely the mature thing to do on both of their parts. Even just the sentiment that she won't hate him if he does release her means a lot. Nobody WANTS to hate their parents.
What do you mean she promised to agree with her brother's edict of releasing her mother!? He knows the empress was mean af and was the cause of her death in her first life, so why would he let her roam free???