Child 18 goes to college, works and lives with sister in Maryland. Child support is being paid to mother in nys. Is that legal? Can money go straight to child or can support be terminated?

How do you know the mother is not forwarding the support to the sister? No money can't be sent direct to child and the age limit in NYS (which has jurisdiction) is 21.

I would check with the sister and see if the mother is forwarding the support. If not, you can try getting the support sent to the sister,

The other option would be to go to court and have the child support stopped. That way you can send the money directly to her.