How can I find out why my sister's partner is in prison?

My sister's partner is due to come out of prison in 2 months, and my family and I are curious to what he is in for; as when he comes out he will be living with my nephew who is 3 and my niece who is 4 months old. He is not my nephew's biological father but he is my niece's. My sister wont let on to why he is in prison, or where the prison is. All we know is that he is in a high security prison and was sentenced for three years, but will be out in 2 months after just serving one year with "good behavior" I'm not trying to be nosey and meddle in my sister's life i am just concerned for my nephew and niece. Help is much appreciated =)