Overbearing step mother

Ok so I’m 20 years old and have been at university for the past 2 years 1/2 years of my life. After the 1sr 4 months of uni I returned home for Xmas and my single father had found a new girlfriend. I was happy, because it was largely just me and him for last 6 years and I was slightly worried that he would be on his own once I left for university. I soon met her and she seemed really nice. But in the past 2 years since we have met she has become increasingly overbearing. She is constantly trying to make me come over and is quite forceful into trying to get me to live with her and my Father. I know this is really nice but I am 20 years old and she treats me like an 11 year old and I find myself in constant situations where I am turning her down and it looks as if I am not wanting to spend time with her family and my father. I would understand if I was younger and barely saw my dad but I’m not and I see him fairly regularly. Also this Christmas I spent 10 days at home with my mother, and spent 5 of them with my dad. Yet as I was about to leave for uni, she tried to pick me up from my mothers and make me come over again! I can’t tell my dad that I’m not fond of her because she’s too nice can I? It’s got to the point where I actually am uncomfortable with her and feel like I can’t go to my dads without her mothering me! also she says things like her children (which are no way related to me) and I look/act alike. Is this weird or am I being to ungrateful/introverted?