I own a duplex and about two months ago I rented one of them to a single mother even though her references were not good. I decided to give her a chance. Shortly after she moved in, the next door tenant informed me that my new tenant had a dog. The lease agreement clearly indicated no pets were allowed. I talked to her and told her that I didn't appreciate the fact that she violated the contract but now that the dog was in, she could keep it as long as she paid pet deposit. She agreed however she has not paid the deposit yet. Couple days ago, again the next door neighbor contacted me and informed me that she had to call the police on my new tenant due to ongoing domestic dispute. She said that there were constant arguments/fights inside and outside the apartment and that it was getting really ridiculous. The lease agreement lists the mom and the child as the only occupants. Even though the tenant did tell us at the time of application that she was separated from her husband and in process of working things out with him, she never formally informed us when her husband moved in with her. I signed a one year lease with this tenant. Can I terminate her lease based on the fact that she got a dog and then had her husband move in and is disturbing my other tenant with her domestic disputes? Thank you so much for the advice.

Yes, you can and there should be a clause in your lease stating that you have a right to terminate the lease if that happens. If not, you can give her x amount of days to perform in writing, have her sign it, and if she does not perform, then start the eviction process.

As long as you leave a paper trail (get everything in writing) and follow the rules concerning tenant notice in your state, you are safe.

You can also run this problem by a local property manager and get their input on the best possible action. In return, give them a free coffee.

... Can I terminate her lease based on the fact that she got a dog and ...

I believe you may have waived the "no dog" part.