What do you do when your pets do not like each other?

I had one dog, Max, for many years. He was an awesome German Shepard mix. When he got older, I couldn't walk with him anymore so I got Tinlee a Terrier mix. She was fine with Max, but when he died I got another dog, Barbie, to play with Tinlee and they played all the time.
Things were good for a few years, but then when I had a baby, Tinlee started attacking Barbie. It was strange. I think Tinlee didn't like the fact that I had a baby and she took her anger out on Barbie.

Years later, I still have to keep them apart, because Tinlee has tried to kill Barbie. When I separate them, Barbie does something really stupid--she tries to go back to Tinlee and tease her. I don't know what to do so I just separate them. It is really hard to live this way. Barbie gets along with all the other dogs. Tinlee, well, I don't know what happened. I think she wishes she was the only dog.