How to Get Custody of Neglected Children

My question involves a child custody case from the State of: Pennsylvania
My sister who is always out with her youngest child's father and is never home and whenever she is still does nothing for her three oldest children.
I along with my boyfriend are currently staying at the home she rents from my grandfather and the time I have been here as basically been the only time her children are cared for. Our mother is currently on the verge of calling CYS on her because the kids are not fed until late at night, their clothes are almost never washed, and they are typically not allowed to get a bath the only exception to these are whenever myself or my boyfriend do it or when their mother isn't home.
The only reason she is even still at the home is because of the children. I have been discussing with my mother about going for custody of at least the 3 oldest children (age 14, 9, and 6.) I am currently a cashier (as is my boyfriend who is also a father of 2) If my sister were to be taken out of the picture I would then most likely be able to rent the house meaning I would have ample room and also not have to take the kids out of basically the only home they know.
My main question is what would I have to do in order to take custody? Their father isn't all that much in the picture.
I have already been more or less raising these kids and am tired of seeing them miss out on their childhood simply because their mother still thinks that she is a child herself.
The only issue is that I am only 18 going on 19 whereas my boyfriend is 22. I fear that my age will make everything more difficult even though my family agrees that I would be better off with the children opposed to their mother.

I don't think you have a chance at all.

If the children are so badly neglected, why hasn't anyone called child services?

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Quoting RG7651

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My main question is what would I have to do in order to take custody?

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Get the parents' permission.