There are a lot of questions that I would need to ask you before I could give any guidance here.  But let me start by saying if the landlord rented the house to you and your family - or what ever the relationships are - and was fully aware of the size of your family and the septic has become a problem then no, he can not mke youleave and he has to fix the problem.  It falls under what is called a warranty of habitability.  Good luck.