Sounds like it might be a little too late for my opinion but I would have said not to including her in dinner, but ok to collaborate on a party.

As for this "You know it's not fair that you have him on his Birthday and the weekend after. Have it the following weekend when you them for Fathers Day."

Life's not always fair and I'm sure in the future (as there will be many, many more birthdays) it won't work out so well for you. Last time I checked this is what SHE wanted....

Me-44 (45)
H- 50 (51)
M-'96

S-18(20)D-15(17)D-12(14)

BD Feb 2014 (he works overseas)
home Oct(sep rooms)
(EAs possible Pa's unconfirmed)
insists wants D through July 2015
no more talk of D since