paying family to watch your children

I know many will have mixed opinions on it. As would I just reading a story of someone elses life without having to endure it myself. But im a bit perplexed. Ive been with my fiance now for 3 years. We own a home, each lease and pay for our cars and insurance. We pay everything separately. My first son is from a previous relationship, so he came into my fiances life at 10 months old, and is now almost 4. When we had moved in together, 2 hours away from my parents who watched my child for free, my mother in law started watching him. I understood that my son is my responsibility and ill be paying for anything he needs, food, clothing, daycare etc. Almost as if im still a single mom. I pay my mil to watch him currently, on top of preschool costs because he does go twice a week to school. I was aready struggling but knew my son needed school and i still needed care for other days. Fast forward to now. Were expecting baby #2 in just less than a month. She actually suggested i put my first son in a full time preschool during the entire week because not only are we expecting a baby, but my sil is too, about a couple months after me. So i understood that would be difficult caring for 2 infants plus another grandkid and my son. I totally fully get it, yet i cannot afford full time daycare. My bills already account for 80% or more of my income per month. Thats not including groceries, gas for work, anything extra. My fiance has not helped out with the extra costs. We split the mortagage, utilities, and groceries basically down the middle. I pay my own car payment, insurance, anything for my son -preschool and his mother- on my own. I also have zero idea if she is expecting payments for watching the baby. I also have zero idea who will be paying for that. Anytime i bring it up its "oh well figure it out later". My fiance makes double, if not more, than my salary. So He doesnt understand how im struggling to afford anything. Im the only one saving up for my maternity leave and the medical costs and all the fees that putting my son in full time daycare would cost.

Im well in over my head when it comes to all these bills. Im not sure how to go about any of this. I dont know whether or not i should ask my fiance to help me out some, or his mother if i can pay a little less. Im just struggling. It really makes me want to move the 2 hrs back to my parents just so i can have some breathing room with my finances. But i also dont want to split up or break the family up. My sons dad is garbage and currently has a bench warrant, and no job, for already not paying child support payments for over a year. Since were planning on getting married, and he knows the situation, i feel i should be able to ask for some relief to not feel like im doing this all on my own. He treats my son like his own except when it comes to finances.

Im stressed, struggling and have no idea what im going to do while im on leave not getting paid and cant pay anything else, if i dont save enough.