Why does my sister put up with? i need advice? 10 points best answer.?

My sister is 20 years old and live with me and my dad. Im 17. She has a son ( he lives with us to) by a complete loser ( lets call her bf gerald). So hes worthless. Hes out on bail for drugs and weapon possession. My sister worked her *** off to get him out on bail. Now shes a mess too. Gerald doesnt work (hes 21) , go to school or anything. He doesnt even have a place to live. He stay over his friends house. He calls my sister names and hits her. He calls me names too. My dad lets gerald drive his car and everything. So today he was abusing my sister again (my dad is 60 so he really cant fight a young man that looks like a big a sumo wrestler) and he had my dads keys in his pocket and my sister was scared so when he walked outside she locked the door on him but she didnt know he had the keys. She asked for them back and he said let me in b*tch. So my dad called the police. But he ran off with the keys. Now my dad has to take the bus to work. Not to mention the house key is on there so my sis is scared (if he was smart he would of just unlocked the door).my dad is kinda old and i had to calm him down because he was going to fight him. He just lost his wife of 25 years on feb 7 2011 (who is also my mom) and he has enough on his plate. He has high blood pressure and strokes run in his family. If i lose him idk what i would i do. Any suggestions? Anything will help. Thank u.