I'm so sorry to hear about ur family and ur mom **hugs**. I hope they understand someday. Have u told ur sister how her saying all this is making u feel? She has no right to tell u ur ruining ur life cuz ur not! A lot of people think that way but all ur doin is maturing earlier on in life and taking care of a living, breathing, human being who needs ur love more than anything. Ur sis should come to know that.
Tell ur father soon tho. The more u wait, the harder it is, trust me. He may be rough in the end but it is ur father, he cant get mad at u for long.. He should understand soon. Im glad to hear ur boyfriends parents are there for u

thats good! Good luck in telling ur dad and hope everythings okay wit u and ur sis!