I am going to be making my FIL's 65/retirement cake. It's going to be a dummy cake, as I have to travel 500kms to the party, and don't feel like stressing over bringing a cake~cake.

So anyway, I was going to do a 3 tiered cake. On the bottom tier I was going to use fondant/gumpaste cut outs of all the places he has traveled (paris, madrid, rome, england) and use popular tourist attractions from each place.
The second tier is going to be things he enjoys (his boat, guitar, piano, music notes, etc).
The top tier is going to be his family (here is where I have my problem). His wife died in 89, before I came into the family. He has been partnered with someone since 90. Do I include the wife along with the girlfriend or do I not include the wife? They were married for 26 years before she died. I am also including his two kids (my SIL and my DH) and my kids and myself.

OR
should I just do something completely different for the top tier?

(not due until June, but wanted to plan it before hand incase I change my mind completely!)

I would incorporate all. It is not tacky or anything in my opinion. When my grandfather died, the paper even had my grandmother's name and his wife since '83 in there and both were mentioned at the funeral. His newer wife was buried with her first husband and he was buried with my grandmother. Both families have been together for a long period of time, so I don't see a problem personally.

I agree with kdmoffett, but if it makes you uncomfortable do something different.

how about just kids and grandkids?

sidestep the problem all together

Have you talked to your DH & SIL? Ask them & if they say go for it, I would also ask the girlfriend how she would feel, if all 3 agree then I would do it. If they don't agree then go with kids & grandkids.