There is a wrinkle you may be able to take advantage of in the US, which I recently ran into. If the girls have been raised together, they will usually want someone to take them both and keep them together. Having a felony in your background doesn't preclude you from having custody of your kids, depending on what it was - for instance as a sex offender.
At worst you may be bale to provide a home to the older girl, to start. The county/state also has a vested interest in seeing the child with the parent for financial reasons. What you are doing is called kinship care, whose standards are lower than standard foster care, but you'd still receive some financial assistance from the state/county for doing so.
That natural parent has the legal upper hand, unless the child can be shown to be in imminent danger. Try to cultivate some kind of dialogue with him to allow the girls as much visitation as you can. If he has no other kids, he may very well decide it's a big pain to have one around full time and be amenable to giving you a guardianship.
Good luck, it's a good thing you're doing.