My advice is to get a good handle on your feelings about those issues. It may be a discussion about money, but you'll be reacting out of your feelings if you don't identify and address your fears and hot buttons (by yourself) before you go at it with S.
Beyond that, ask for what you want. Listen. Be open to different solutions, but be firm about your boundaries. Do not try to convince S to see things your way, just make specific, behavioral requests: "I'm unable to work while caring for Lily right now. Would you be willing to pay all the household expenses(or whatever)?" If not, "Oh dear, I don't know how I'm going to be able to afford to live here. Do you have any ideas about how we can make this work?"
