Do you have a power of attorney from your sister or the children's father(s)? Were you appointed guardian of the children by a court? I'll assume you haven't adopted them.
I assume what you found out is that some girl is claiming to be pregnant and is claiming that your nephew is the father. Right? There's only an allegation and not any actual evidence of paternity. Right? Assuming I'm right, neither you nor your nephew should accept or concede that he is the father.
No. It's utter nonsense. Nor are your nephew's parents responsible for anything. The only persons with responsibility for this child, once it's born (if it's born), will be its parents.
For starters, in the eyes of the law, your nephew will not be regarded as the father of this child until (1) the child is actually born and (2) paternity has been established (typically by DNA evidence). If we assume that the child is actually born (the parents/legal guardians of the child-parents ought to seriously discuss the benefits of terminating the pregnancy) and that, after it is born, your nephew's paternity is established, then the mother (or her parents on her behalf) can seek an order requiring your nephew to pay support. No one is going to seriously expect a 14/15yo to have the ability to pay any tangible amount of child support. The court may order a nominal amount. You'd need to consult with a local family law attorney about how the court in your locality will handle this.
At the same time, you need to sit down and have a frank discussion with your nephew about how the stupid decision he may have made (if the allegation is true) is going to impact his life for the next two decades.
