I think jumping to bringing in the police or DCFS is a little excessive. No, I don't think it's healthy for a 12 year old to be having sex daily with a 15 year old boy, but I also don't think involving the police is going to make the situation any better.
You and your husband need to sit down with her, treat her like an adult, and have an honest conversation. Talk about facts - pregnancy, STDs, being emotionally mature enough for this type of relationship, etc.
Explain to her that you are not comfortable with it and think she's too young, but LISTEN to her side as well. Find out where she is coming from and why she is doing it. Does she truly want to, or is she feeling pressured? Does she think that because she said yes one time, that now she can't go back and refuse to do it? There could be so many factors that a 12 year old just might not be able to handle on her own without some help, guidance and support - not fighting - from her parents.
Have a gyno check her out and prescribe birth control - you can decide whether or not to fill the prescription. Better safe than sorry.
As for your husband's relationship with his ex, that's always going to be a tough situation. He needs to talk to her about not putting him down to the daughter, and you guys need to make sure you aren't badmouthing the ex to the daughter too.
Bottom line: your husband is her father. He should involve himself quickly in this situation and talk directly to his daughter, regardless of anything the ex says.