"If we as a famnily HAD to rent privately and council housing wasn't available to us we would be HOMELESS and thats the absolute truth."
OR you would not be having your third child at the age of 23. You would work, save, and have more children when you could afford them.
I realize this is a bit harsh, and I do apologize if that hurts your feelings, but it is the truth.
Your sister got very good and gentle advice for a very long time on this thread. Everybody said the same thing - you are not making much money as a family now, you are reliant on parents and taxpayer, wait a bit, study, work, live your life, save to afford your own place and provide for your children, then have another baby. You are very young, there is no rush.
Unfortunately, all she wanted was some hugs and support. Which is when/why I said "You might like BabyCentre better". And she probably would.