Can foster parent (grandmother) voluntarily transfer her right to claim child as dependent to biological parents via Form 8832?

Our 17-year-old child was court-ordered to foster care in 2017. My mother-in-law legally become the permanent foster parent for our child. She had physical custody of our child for all of 2018. Does this make her the custodial parent and my wife and I the non-custodial parents?

Also, she had offered to let us claim our child as a dependent for 2018 taxes (to help us out financially). Could this be accomplished via Form 8832, or is this not possible between a foster parent who's also a relative?