I know to a sertain extent what you are going thought!!
i know that it most be hard to accept this man as i gather you were very close to your dad, i hope he would want your mum to be happy again and move on?
i allways go wit Rios choice in a man especielly, if she does like them its a good sign that i should be awear of them (not desgard them totally just be awear)!
you could give him (and his sprog)(and cats) a small chance etc
if there is are any signs of violence towards any of you (dogs included) get rid of them no second chances!!!
also if there is any word of "that dog must go" well you know what to do!!! they were here first!! if any thing it should be the cats that go if there is a problem!!! the cats wont pine after them but the dogs will after you!!!
Let us know what happens please
Ky and Rio