Dad remarried 12 years ago and has been stricken with dementia for the last couple years. His wife and her daughters have kept him away from me.

This is a sad and not too uncommon situation, judging from other people's questions on this forum. You have a right to see your dad. Did you have problems with his wife before his dementia started? If not, this is really strange. If so, she and her family may be taking advantage of his dementia to keep you away. Either way, I'd keep trying to nicely convince them that he needs to see his children. You could try to get a mutual friend or clergy member to intervene.
If they totally bar you from seeing him, you may want to contact social services and see if they can guide you. Of course, an elder law attorney could help, but that's extreme is you don't need to go that route.
Take care,
Carol