The OP shouldn't start making deals; she should just ask them to explain, via solicitor, their legal position ie ask to see their hand (I suspect they don't have one).
Parps is right - any attempt at "fairness" will be viewed as weakness.
OP and her DH were just fortunate to be given this money - and unfortunate that the GF died with the 7 year period, which made this cash-grab attempt possible. Under NO circumstances should she and her DH start negotiations.
OP take the advice of people on here and contact a solicitor - I would get one who specialises in this area if you can, as I have a feeling that these things may be on a case-by-case basis depending what other provisions are in the will, and, of course the size both of the gift and of the estate.
Obviously the GF didn't explicitly exclude the gift (probably didn't even give it a thought), but neither did he say "Except for LiskenlyDH and Liskenly who have already had their share" - so it's obvious to me that he didn't consider the gift "their share". However the law might have her opinions - this isn't something to be decided by well-meaning people with no legal training.
Don't try to keep the peace or maintain good family relations - that ship has sailed when the other family members started making demands: If OP'sDH gave in to them - they'd be crowing that they'd won and would feel justified that they'd stopped him ripping them off (and wouldn't come out of the woodwork until another relative died) and if OP'sDH didn't give in to them, they would nurse their hatred for centuries. Families are a bugger in situations like this.
Greed - the gift that keeps on giving.