I need advise!

So my dad has always been here and would do anything for us, but I’m getting so annoyed now…. So my dad is with this woman they been together about 7years since he has been with her he hardly comes over now they run a pub, he doesn’t do anything for us, I have a son who he doesn’t come and see or take out, but his Fiancé can go on holiday ect ect…. My last straw was Christmas Day when he said there really busy but couldn’t pop up to see his grandson for an hour on Christmas morning…….. my Nan has COPD, she gets taxi to do shopping(I help her mostly) my dad can take his fiancés family members shopping but not his own mother everything revolves around this woman and pub, I went mad ages ago because she posted a picture of her nephew and my dad at the beach playing, but couldn’t take his own grandchild, what would you do any advise would be great