I agree with everything said here about confronting your ex, documenting all bruises with pictures and descriptions about when, where on the body, what happened, etc. If need be, get your pediatrician involved in documenting the injuries since pictures won't do much good.
Also, about your daughter's defending themselves. Teach them that "hitting" and "protecting" themselves are two different things. Explain this to your ex too. That you are going to teach your girls to YELL when they are being hurt. They are going to "protect" themselves by hitting back or running away. And they are going to tell on this other child EVERY time she's mean. And that you expect your ex to intervene.
I went through something similar in our gym's child care. One of the other boys was mean to my little girl. His mother was one of the 'teachers' and did little to intervene before he hit, pushed, etc. So I taught my older daughter to stick close to her little sister when he was around. To YELL at him "go away", "leave her alone", or get physical to defend her sister while yelling "STOP HURTING MY SISTER!" This way she wasn't "tattling" on this boy, she was drawing attention to herself and him to get the adults attention. After a few "confrontations" with my older daughter the boy would steer clear of my kids. And it taught my kids not to "tattle" or always depend on adults to come help. They learned early to take care of each other and stand up against bullies.
I don't usually advocate being physical, however I do believe in defending yourselves/siblings. And if the other kid "happens" to get a bloody lip, well so be it. Maybe if this other little girl gets hurt, maybe then your ex and his girlfriend will be more attentive when the girls "play" together. I have learned from experience that when the tables are turned on physical kids, suddenly their parents are more willing to get involved when it's THEIR baby who's getting hurt.