1. Daphne Mandaro should use good reliable birth control pills daily.
If my boyfriend did get me pregnant, could his foster parents (who are also his sister and brother in law) keep him from seeing me, or my parents do the same? Is there any laws preventing us being separated?
If either of you are minors .. You bet they can separate you. IF you get pregnant, even if you do get separated, it would not diminish his or his parents legal obligations for child support.
If you are a minor and he is an adult ... His parents won't have to bother separating you .. He will go to jail so they won't have to bother.
IF you are engaging in irresponsible unprotected sex .. They SHOULD separate you long before you get pregnant .. Period!
You didn't mention your ages.
A baby is a 24/7 job and your lives will be dedicated to bringing his/her life up, not much time for either of you within that. There will be less focus for you or your partner which sounds like a relationship which is based on no good grounding ie maturity, finance (babies cost a lot), a roof over your heads. If anything is going to split you up it will be having a baby. People who responsibly want to have children because they want to put that babies life first, understand the large commitment that demands, and make a grounding for it. It sounds like you are putting very little thought into any of this and the quality of life you will afford your baby. Babies don't save relationships they put tremendous pressure on them.
How old are you? If you're both adults, this shouldn't be an issue. If you're too young to be having kids .. Then you have to expect people are going to give you grief about it. Until you're both independently ready -- financially and emotionally to have a child .. Don't have them. That makes life for everyone much easier.
If you are under 18, quite possibly.
But realistically, you don't have to worry about it---Being a young mother is a 24/7 job and your boyfriend will be working 2 jobs to support the 3 of you.
You wouldn't be seeing much of each other even in the best of situations.
Pepper pot gave you a more detailed comment.