Grandmother overstepping her role to my 8 year old

What is she doing that you don't want her to do? Specifics would help people who can give you good advice

The great thing about having grandparents is to help get a good balance of thoughts and ideas. No offense but having everything just your way is an imbalance. It's not about power, you're the mom and you rule but lighten up. I agree that unless the mil is doing something that can cause harm to your child, let her enjoy being with your child. Do you know how many moms would love to have someone helping? Embrace what you have. Learn to compromise. The child will benefit from it.

What has she done specifically?

Hun, understand this.....grandmothers have already gone through the discipline and authority stage, grandchildren show up and they want to spoil, this is what grandparents are famous for but don't interpret this as though she wants to step in as the mother, she has already done that role, cut her some slack, and you provide your rules at home....and let their time with grandma be specially different....

Is she letting them play with matches & watch porn? Or is she giving them candy before dinner? It would help to know what she's doing.

I would over look it and realize that not every child is blessed enough to have doting grandparents.. I also would not allow myself to get all worked up about it. You know who their mother is, they know, and so does she.. If she wants to spend her money on them, it's her money.. If they get used to her buying them toys and clothes every time they are with her then she will have to deal with that herself.
Just because she makes comments to you about how to raise your children doesn't mean you have to take her advice, nor do you need to be concerned if she has a problem with it. If you don't want them to spend a weekend with her when she calls, say no. Why does it have to be an issue.. Just say no. If she has a problem with that, too bad..
Look at it from a different angle.. You don't HAVE to be annoyed or frustrated by her. No one can make you feel or do anything unless you let them..

Quit letting her spend time with them unless she can follow your rules?

^^^admckenzie: I couldn't agree more! I absolutely adored my grandparents & feel like a big part of me has been missing since they passed away 3 yrs ago. I'm lucky to have had them in my life for as long as I did, and would never dream of denying that type of bond to my children or their grandparents. Remember, different doesn't equal wrong, it's just different.

Why isn't her son handling this?