she doesn't give your needs/feelings/space same priority she awarded to him by whatever reason, and she evidently puts enough mental effort to downplay his attitude/whatever he did in the past. she might keep some sort of internal rationalization in her head that she doesn't outright voice in your arguments.
there might be a partial layer of stronger bonds between mothers and sons vs mothers and daughters - it's a stereotype etc but it's pretty accurate overall.
I can't give out any advice in this situation, but given how much external perspective I've gained on family hypocrisy over the past recent years, there's very little chance for them to recognize that you indeed have a very serious and a very real reason to treat him the way you do, and act on that realization.
it's either an ingrained internal double-standard for her precious first born, or crystal clear cynicism, and I've seen examples of both.
there're ways to do that (a very serious conversation over your own view of what's going on and your clear disapproval of her repeating patterns of condoning him is a start), the problem is your best bet often times would be either developing coping mechanisms if you are otherwise better off staying with your family in your circusmtances, or just outright packing and leaving if you can afford the financial burden of moving out and have savings to do so.
based on your side of story, the age and sex disparity, your can speak out, but there might be no one listening.