Need help w/ Tenant problem - WWYD?

[Marci Stein]

Family with 4 young children have been renting for two years . Last month I told them I am not renewing the lease (up in July 4 ) and they said they’re not moving ( no $) and I’ll have to evict them . 
yesterday they told me water is leaking from toilet to tub . 
today I had the septic pumped and was told even though it is now empty , there is a ton of grease in the lines and the plumbing still will not work . 

tenant denies putting any grease in the pipes .

Lease states he is responsible for any plumbing problems other than regular maintenance . 

The last pumping was two years ago right before he moved in and he clearly had a working septic tank for two years. 

I’m thinking if I do not fix it he will be forced to move but then the town might condemn my house and I have to pay for his relocation . 
I feel stuck - advise please?