You are right in many respects Indiana Rose!
Actually, the fact that everything about this situation IS everyone's business contributes greatly to this problem.
But, the current facts of the matter are that this is all open and has been made the OP's business.
How do the mother and the brother and SIL think it is going to benefit the OP by openly discussing (flaunting) all of this in front of the OP, while the mother stands there with her hand in the OP's pocket.
OP, this is actually an important point that we are revisiting here.... Just how are you sure that your brother is not paying anything?
How are you sure that your mother picks up their kids every day, and does not ask for gas money... I can only assume that they have no problem discussing this with you, or in front of you, right to your face.
First, it sounds like there are no healthy boundaries in your family.
Second, it sounds like they are very happy to openly (subtley or not) flaunt this at you.
WDW4me is right, it is good to come here and vent, and to 'chew' on this and digest it all and mull it over.
One should NEVER be made to feel bad or guilty for their feelings.
Only you can decide if you are being blatently slighted and disrespected.
Only YOU can decide what you are comfortable with.
Personally for me ( I can only speak for myself ) I would be able to deal with some favoritism... the clothes, books, etc..... That, I think I would be able to let go of that completely. I would not be holding any 'accounting'....
But, I would not be comfortable handing cash over to my mom while knowing that I am basically enabling this and watching it happen, and hearing all about it.
Your mother has every right to 'do' for her grandchildren. Whether it is 'equal' or not.
But, when it comes to asking you for cash... to me, that is the kicker.