when is it a good time to come out to my parents ?

Stop being so selfish and start acting normally. You have no right to wreck eveyone elses live in your family by bringing this shame upon them. Get a boyfriend and stop being so greedy. Have only normal sex, get pregnant and start a family. Make your family proud of you and not ashamed.

I see no reason why you should come out in the first place and it doesn't sound like a good idea in your current living situation anyway.

Keep your perversion to yourself. It must remain a dark, shameful secret not be paraded around in the streets! 

EVerything depends on your age. How close are you to graduating? Yes rooming with your stepsister will not work if you come out as bi. But how are your folks? Will they freak it? If you feel uncomfortable or unsafe keep this news to your self for as long as you can. In the meantime reach out to the gay center and get strong inside. It is really important that you know you will be able to handle your situation when people give you a hard time.

If they’re trusting people and not homophobic, try just telling them instead of your sister but if you can’t trust them to not tell her then maybe you should wait until they get a little more open minded.

Gays burn in hell