My boyfriend and my sister will be the custodians of our children if anything happens to us-per our will.
I see no issue with it as long as the person(s) you choose is good with kids and your kids are comfortable with them. The person should be capable emotionally, physically and mentally of supporting and loving your children as if they were their own. Not AS their own, as they WOULD love their own AND they should be willing and able to help the children maintain relationships with ALL family members and loved ones if something were to happen to you as well as ensuring that they keep their religious backgrounds as you've taught them
"Love As Thou Wilt"