I don't want to spend the weekend with my wife, her mom and her grandparents
Why don't you want to spend the weekend with them?
I think I need some time for myself.
Spending time with family can be too much sometime. It is okay to sometimes spend time apart. It will help you enjoy the time that you spend time together.
Sometimes being around so many people can be exhausting.
I agree with you, and I think that you should tell your wife how you feel. It isn't necessary to always spend time with your family, and spending time apart can be healthy.