No, probably not until that person's 18.
No, they are not yet an adult and cannot have custody of a younger child.
A step-parent has no legal rights regarding your child. The biological mother has visitation rights and other rights when the child is in her custody.
yes
sole custody
If court ordered, yes.
The parents have to go back to court to file a modification of the custody order. They should also terminate any child support order that obligates the father to pay child support
That depends on the custody order, but she can file for a change of custody.
See related link below.
No. A ten-year-old is not a suitable custodian for a younger child. Anyone who leaves their child in the custody of a ten-year-old is guilty of neglect.
Only with approval by the court of a Motion for Custody of a Child In Need Of Care.
You petition the court for custody of the child.