What do you do when a grandparent visibly favors one grandchild over another?

There's not really much you can do about it. Just try not to draw too much attention to the situation especially if the other child hasn't really caught on. As a parent just try to focus on your own parenting skills which include treating all your children equally. :)

Tell her how you feel. I am going thru this right now. My sons are 6 months and 19 months and I asked the grandmother to babysit the other day she is like well I have to bring so and so. I said why can't you let the parents raise him and spend some time with my kids? She didn't like it but she got the point. She favors this child because the parents aren't the best but when they try she still doesn't like it and tries to keep him from them. She wouldn't even spend the 2 weeks in GA with her other grand kids that she was supposed too because she wanted to come home to see this child. I was mad and they wasn't even my kids. Now she is doing the same thing to my kids and makes me really angry. She yelled my son the other day because she thought my kid hit this grand kid on purpose and he didn't. She babied the other kid for 20 minutes. I told her not to yell at my son I saw what happened and it was clearly and accident. Your just going to have to speak up, that is what I had to do. Yeah it might be rude to do so, but my kids are still young enough to where they haven't realized it. I would rather have to tell her now what she is doing rather then my kids finding out the fact grandma favors someone else over them.

Nothing. People are idiots. All people. Even grandparents.

people = idiots

grandparents = people

Therefore

grandparents = idiots

Well there is nothing to do about it. All i can say that it's not very nice to show out that you like one more than another.