The first thing you need to do is to get the father's parental rights
terminated involuntarily. In Massachusetts, according to Statute: Ch.119 § 26(4); Ch.210 § 3(6), the circumstances that are grounds for termination are : Abandonment
or Extreme Parental Disinterest; Abuse/Neglect; Mental Illness or Deficiency; Alcohol or Drug Induced Incapacity; Felony Conviction/Incarceration; Failure of Reasonable Efforts; Abuse/Neglect or Loss of Rights of Another Child; A Child's Waiting Sexual Abuse
; Failure to Maintain Contact; Failure to Provide Support; Child Judged in Need of Services/Dependent; Child's Best Interest; Child in care 15 of 22 months (or less); Felony assault of child or sibling; Murder/Manslaughter of sibling child. Based upon your explanation, it appears that several of the grounds exist for you to seek the involuntary termination of the rights, through your sister of course. If your sister is incapable of doing this, then you first must seek to have a guardian/conservator placed in charge of your sister's affairs and then bring the action to involuntarily terminate his rights (you could be appointed temporary guardian
/conservator for this purpose).
Once you have terminated his aprental rights, then you can apply to get temporary custody from your sister. This type of complex situation is best handled by an attorney and if you cannot afford one you can get a low cost or pro bono attorney through legal aid or the Massachusetts or Boston Bar Associations.
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