How do i deal with parents who are putting to much pressure on my sibling?

Talk with them and discuss an agreement to come to.

I have gone this before, but this is what i did, they wanted her to do this and that  and i was sick of it. so one day when she was told to clean the bathroom i said no, i was sticking up for her cause it was rude. then i told mom and step dad why and they finally understood. maybe talking to them will help and explain why will help you and your sibling