Kids don't want to visit grandma anymore

I am having a problem with my mother. Growing up, I was not close to her and I have made peace with that in my life. I have two sons and I have let them have their own relationship with her because I don't feel I need to ruin things between them just because she and I never got along. However, they are not wanting to visit her much anymore. Mostly because she tries to do things that disregard the fact that I am their mother. She has always done this but now that they are older, they are really getting the whole picture. For instance, she proposed that she take my newly sixteen year old son to get his driver's permit. I told my son I wanted him to wait until he was 17. Not because he is an irresponsible person- he is far from it. It's just that it has been shown that states that move the minimum driving age up have significantly lower death rates of teenage drivers due to certain developments in the brain that are not there when they are 15 or 16. She kept pushing and tried to take him anyway- even though he told her not to worry about it because I had already discussed my concerns with him and he knew he had to wait.

She buys things for both my sons and even though kids like to get things, both my boys feel kind of put upon because even if they say no, she buys it anyway. Some things are expensive, some are not but both my boys know how I feel about earning things they want rather than just having it handed over to them. Even though they are thankful, they still feel rather uncomfortable accepting things like this because she often laments about how poor she is and doesn't have the money to fix her car or take her dog to the vet. Sometimes I feel she does what she does just to take a jab at my authority. I also think it irks her that my kids and I are so close and she wants to buy their affections. She seems to get angry that it doesn't work.

What she doesn't realize is that running over my kids like she does doesn't make her a saint in their eyes. My youngest does not go to her house very often anymore. My oldest visits her more often but it's more out of a sense of duty. I rarely talk to her myself. My kids are not mindless little blobs. They are well aware of her shenanigans and even though they do love her, the fact that she doesn't listen and does what SHE wants is starting to sour them on her. She thinks that I am the one putting thoughts in their mind to not visit. I'm not.

They've listen to her talk rudely on the phone with my sister about my nephew's wife whom she has never even met, she used the term 'wetbacks' when referring to the Mexican couple that lives upstairs from her and there's been other things that have put her in a dim light with them. They love her but they don't like what she's doing. I don't know what to tell her because she is never wrong. The boys don't want to hurt her feelings but she gets bent out of shape when she offers for them to come stay with her and they turn her down.

I don't want to tangle with my mother because the words are always harsh between us. I don't want my kids feeling stuck in the middle but I don't know what to do for them. She doesn't listen to me, she doesn't listen to them. She simply doesn't listen!