My mom's siblings are so mean to her and my grandparents

My mom has 2 brothers who I feel have been jealous of her ever since our family became more financially well-off. My mom is always thoughtful of them, always goes out of her way to buy them food, take their kids out to have fun, and every time we go on vacation she always gets them souvenirs-I know that my mom has good intentions, but all of this I think has only ever been seen by them as my mom trying to show off. All this time, I've never seen them do any nice gestures for my mom, which really is upsetting because my mom doesn't realize this but I've been seeing this all along. But because I was young back then I didn't dare to say anything about it. These 2 brothers also always try to conceal the amount of money they have from their parents (my grandparents) to avoid having to pay for any medical bills or help out with living expenses since my grandparents are both retired. My parents are normally the ones who cover my grandparents medical bills-even though we don't mind this, it's really upsetting to think that even when my grandmother had to have surgery, my uncles just disappeared so that they wouldn't be asked to help split the cost (smh). This really disappointed my grandparents and my mom. Recently something happened that caused my uncles to be really mean to my mom. They even used my grandparents, trying to get them to scold my mom by saying things like "you don't love us, you only love her(my mom)". Of course, my grandparents are reasonable and refused to do so. But because of this, my mom is really hurt as she now sees that her brothers don't even think of her as a sister. They always try to get things out of her, like money borrowed that they don't return, etc. And now this. I'm really frustrated to see this, I don't understand why they are so mean to their own sister? How does jealousy like that grow within siblings? What can my mom do about this? She's tried to talk to them, but one brother didn't even want to answer her calls and posted some nasty comments about her online. It's just really frustrating for me to see because being a young member of the extended family, I can't really speak up against my elders. Sorry for the rant, and thank you!