A good simple and easy way to keep your sheets from twisting in the washing machine is to get some mesh laundry bags and put the sheets into the mesh laundry bags and then wash the sheets while they are in the mesh laundry bags.
The mesh laundry bags will allow water and soap to flow through the bag and clean the sheets while keeping the sheets from becoming tangled up.
I have always washed my bed sheets by putting them in mesh laundry bags first and then put the mesh laundry bag into the washing machine along with the sheets in the mesh laundry bag and I never had anymore trouble with sheets tangling.
You could also tie the sheets up before placing them in the washing machine so that the sheets won't be moving around loosely in the washing machine so they shouldn't tangle up.
But I would recommend using mesh laundry bags for the best results in keeping your sheets from twisting in the washing machine.