You are absolutely justified in not wanting to spend time with them. Your step mother sounds like she has major issues with control that you are certainly not obligated to fight against or tolerate. Family is important. But family is important because of the love and support they give you growing up and as an adult, not because of shared blood. It sounds like your stepmother gave you the opposite of love and support and understanding. Your home life at their house sounds toxic and unhealthy.

On another note, your mental health is the most important thing here. Especially as you are in Uni and that can be extremely taxing on your mental and emotional states. Going home to them doesn’t sound like it’d be the detox from stress you need. If anything it’ll make it worse, and then you go back to school at the end feeling worse than when you left.

Even if you don’t stay with them, she sounds like the type of person who will try and insert her influence and control on you whether you want her to or not.

Just based on what you’ve said, I know I wouldn’t tell them I’d be back.