The Constitution has been interpreted by the Supreme Court to give parents a fundamental right to raise their own children in the way they see fit. However, in extreme circumstances that right can be superseded by the state's desire to protect a child.
Therefore, in most states you have to prove that either BOTH parents are unfit to have custody and/or that there are exceptional circumstances that exist that warrant you having custody of your nephew. Most people think that there "exceptional circumstances" in their case but this standard is very, very high and many courts require you to prove actual harm to the child if the child were to taken away from the third party custodian (which assumes that you already had your nephew with you) or some type of expert testimony about the harm that your nephew is being exposed to.
From what you said it sounds like your sister is a good parent who is making a bad choice to stay in an abusive relationship. If you pursue this, you'd be taking on BOTH parents. Is there anyway you can help your sister get the baby the shots and clean out the mold in the house?
You do have other options if you feel your nephew is in harm's way. You can call Child Protective Services and CPS will perform an investigation to ensure the child's safety. Think long and hard about this because you must weigh the safety or your nephew against the probability that your sister may try to cut off your relationship with him once she finds out you are pushing for this.