YTA. It blows my mind that you can’t see why someone who cares about you would be worried about this relationship. Not only is there a ridiculous age gap, this is someone who was your teacher not even a year ago.

He knew you, a minor, had a crush on him and ensured that you spent extra time together. That is highly questionable.

What a 35 year old has in common with an 18 year old is beyond me. I get that by law at 18 you are considered an adult but you are literally still a child. If you were 21 then ok. But 18????

I feel bad for your sister. For calling out an obviously questionable situation she gets cursed out and ignored by you. How nice. YTA.