Well done, despedina.
One further thought I had - are you part of a home parish where you are? I will suggest that you have a Mass said in the New Year for your late FIL, and invite your MIL to the Mass, to a weekend or overnight at your home, and for a nice dinner that honors your FIL. Just you, your kiddos, DH, and MIL.
If not in your home parish, then in your MILs, but the meal to be at your house afterward.
I can't remember who posted it (sorry) but it's absolutely correct that all Masses offered on Christmas Eve and Christmas Day are part of the Nativity of Christ, and cannot be offered for individual intentions.