how to split living expenses

I'm going to be moving in with my boyfriend & his 15 & 23 yr old daughters (my kids are grown & on their own). My house isn't big enough for all of us so we are going to RENT something together & I'm selling my house. I make about $20,000 more than he does but I'm struggling with how much I should contribute to the household.
Should I kick in as much as he does? 1/4? 1/2? OR should all of my income go into a joint account. When he lived with me for a few months, he paid $150.00 a week (although not consistently).
Their mother doesn't pay anything toward the daughters. I don't want to pay to finish raising 2 more kids. Contributing to the household is one thing but the financial responsibility of the daughters is something that I do not feel is my responsibility.
HELP!