Is it normal to use your child as a weapon to punish someone?

My older sister and I have had a pretty rocky relationship, getting extremely close as teenagers but getting involved with lots of drugs that eventually tore us apart. She and I both suffer from bipolar disorder, but i get therapy and she refuses she even has a problem.

Last Nov. she had a baby girl, though it was an unplanned pregnancy with the man who would become her fiance, and that brought us closer, as I became extremely attached to her child, and became her primary caregiver from 4mos - 9 mos. I was very in tune with my niece and provided the best love and care for her - when my sister would be unaware that she had been in a dirty diaper i would be the one to tell her. One time we were hanging out and I had to tell her three different times that her child was wet, and she would say,"how could that be, i just changed her." or, "no she's not-" just so she wouldnt have to change her.. I also made her aware that she was feeding her baby spoiled milk or from a dirty, smelly bottle and other little things like that. Not to make my sister feel stupid, but to make it better for my niece whom I love dearly.

My sister and her fiance were smoking weed and drinking still, sometimes my niece would spend the night at my house so they could go to a concert and trip on acid. i thought she would be safe with me so i never minded, although they knew i did not approve of their drug use. My sister loved her fiance dearly, but he cheated on her and that was the end of it.

At first, i only knew her side of the story, so I was extremely upset at him and supported her when she went to court to file something that would make it so that he wouldnt be able to see her or the baby til a court custody could be made. she claimed he was putting them in danger because he had a previous dui that had nothing to do with now, but she thought he would drive drunk with the baby in the car. anyway the judge threw out the order of protection because he was posing no real danger to her or the baby, but gave my sister the right to decide when he could see the baby until custody.

When my sister would ignore him, not even answer any emails, and refused to let the dad see the baby for weeks - I said I do not approve of that, and suddenly she thinks im taking his side. i know a kid needs a father and it must be traumatizing for my niece to suddenly not have a dad. my sister thinks i royally betrayed her, so she quit having me watch her, and won't even let me see my niece now. the baby is being watched by multiple caregivers, which concerns me because she is just starting to develop trust or mistrust. everytime i call my sister, she wont let me see my niece, its been 3 months now. i dont know when court is, but i know she hasnt let the dad see her either. i have cried so much over this because i miss that kid so much.

whats wrong with her? she calls me psycho for missing my niece so much... even though it was my sister who's stayed a pysch ward before, not me.