Baghira has long forgotten that he had a sister.
IF you gave Lily back to him it would be an introduction as if they were strange.

If he wants to create bonding between Baghira's mom and Baghira they must be neutered. If not, the mom will only see him as a tom and when she's not in heat she won't want closer contact with him.

Lily and Luna are so close and very good friends. It would be not good for them to separate them.

All these arguments are true. They may help you to take your emotions serious: I am sure you don't want to give up Lily. It is ok if you love Luna just a wee little bit more. Everyone on this board will know I love Filou just a tiny little bit more and Tigris is his daddy's boy......

Lily is now a teenager and not so quiet and social but she's happy with you and she's your cat

And I agree with everyone: it's just not fair if her poor mom gets babies every year. And regarding Baghira: do they know what it means to have an un-neutered tom in the house? Do they know that he will forget very soon the other cat is his mom?