What would you do with a relationship that keeps being interrupted by the father of the children.?

every time their is a problem she calls him. But she constantly tells me she detests him greatly, and now she plans to let him move in with her because she could not get the rental that she now has unless he helped her with the rent, so his name is on the lease. She tells me that she maybe pregnant with my child yet she continues to let this man in and out of her life. how can i be the man if this continues. she claims she had know choice in the matter as she was being evicted from her sisters house this was something that i was against in the first place. but instead of asking me for help she asked him instead maybe it's because the kids are not mine. but anytime the father comes around we can't have any contact for fear he finds out then he won't help her out with his own flesh and blood. To me this is some sick stuff that i've been caught up in what do you think i should do.