My husbands dad owes his mom back child support that she let slide, it's a lot of money she's let him not pay her for both my husband and his sister... My husband is over 18 now obviously and someone told us that this money that his dad still owes for him is HIS MONEY and he could potentially take his father to court and get all the money from him... is this true?

The money would likely be owed to the mother since she supported her son and daughter. It may be too late to collect it depending on laws where the support order was entered. Might be worth the mom looking into if the father has the money to pay.

The child cannot try to collect; only the parent to whom it was owed.