My sisters fiance is mean to me?

she is my only sister. she has been with her bf/fiance for 4 years. (i say fiance because they have said they will one day get married, though they havent set a date). he is so mean! he gets drunk with my dad at night a lot and he is mean to me and my little brother. he thinks we are spoiled brats. he teams up with my other brother and they are so mean to me. i was laying down and my brother through a basket ball at my head (i wasnt even looking, i didnt even expect it!!) and i start to cry and my sisters bf says "show me the blood!! BOO HOO your going to cry then show me some BLOOD" and then he high fives my brother and they say "good luck with your migrane." and he s in and does mean stuff to me all the time. what do i do?