Rant: my mom treats me like I'm my dads second wife

My parents have been separated for 3-4 years now. She does not live in our house and is staying at her lovers place. She does the grocery shopping once a week usually the last day of the weekend  and just chooses to buy tons of frozen meat/fish/chicken. I've come to realize over the past few months that frozen food actually tastes very bland.. Re freezing really loses any taste in any meat. So I've been buying some myself and cooking them the same or next day for myself, sister, and brothers on weekends because that's when we gather. No she doesn't eat here or stay here at all. 

We know that she uses the grocery money that my dad gives her as an extra salary so it's obvious she buys the cheapest of everything sometimes in bulk just to have money left over for herself and wherever she's living. 

I told her that since she can't be bothered to go and buy certain things every now and then for us (her kids) then I should be given some of the money since that's what it's for (us). 

She started saying things like don't start any issues with the money and how much are you spending. Mind you it isn't as much as she thinks especially for one or two days in the week. 

So basically she hates me because I can convince my dad to do anything while she has to struggle because she's having her own mid life crisis by cheating and wanting all the money she can get from my dad..

All this was through what's app voice messages. There are more but I'm not going to listen to any of them because she just wants to be selfish right now..

I'm adding this anonymously because I think it's really embarrassing and just want to hear from anyone out there..