You should take the defence of doctrine of delay and latches.
As they did not came up for 10 year now they have no right to come forward and claim the property. As limitation period for civil suit is 3 years.
If their other family members signed then it is not a defence on their part that they both were not aware of this transaction.
As you have made payment and taken possession as well as completed sale deed and they never objected for this transactions for last 10 years, they cant really defend their ignorance and get the property.