I am very sorry for the loss of your daughter, and I know this is a tough time for you and your family.
I don’t know the nature of the relationship between you, your current wife, and your ex-wife – but since the divorce was so long ago and your current marriage is also well established, I would say your wife should attend the funeral.
As for where she sits, if not next to you, then behind you. You are a grieving father and entitled to have your wife by your side, but then your ex-wife is also a grieving mother, and her feelings must be considered. If you know of any reason why she would be upset by having your wife next to you – then it is best to have her behind you.
Broken families can make for difficult funerals, but ultimately you and your ex-wife will need to decide what is the best way to honor your daughter with the seating at her funeral.