My parents complain about each other to me, and I don't know what to do.

The best solution is to be honest with them, and say that it makes you uncomfortable. Respectfully.

You have every right to inform them that their complaints about each other are bothering you. They have a problem with each other but it's not affecting just them anymore, but you. As your parents, they should see that you are not their therapist or venting machine. Inform them about this, perhaps individually at first.

Tell them that you feel uncomfortable and would prefer them not to talk about your other parents in a negative way to you

Try to just focus on yourself and act like you are interested. It's okay to not know what to do. Believe

Maybe you should ask them to be honest with each other to help their relationship grow. You can also inform them that you feel uncomfortable when they complain about the other to u.

Try talking to them individually and telling the how you feel. Also it might be even better to talk to them together once that is done. So they both know how you feel and where you are coming from.

Sit down and have a family discussion on how they can deal with their emotions instead of putting stress on you

I would tell them that whatever issues they have are between them and you don't wish to be included. I would also reassure them that I love them both and for them to complain to me is very hurtful.

I heard mine do that once in a while as well. I never told them, but I wish I told them that it really bothered me because I really didn't want to be annoying..