I think she asked Ennis to her wedding out of curtesy and respect to her father.......truth is, her mother probably told her "you better get out there and let your father know about the wedding and invite him"....truth is, she probably did not think he would be able to attend anyway.....
It was obvious that Alma Jnr and Ennis had not had a lot of communication since she had met her future husband, Ennis didnt know what he did for a living, or even his name, so the father and daughter were not THAT close.....and that doesnt mean there was any friction with them, a teenage girl would be spending more time with her friends and boyfriend......
I think Ennis did a bit of soul searching, listening to Alma Jnr talk about how much she loved her guy.....and saying he would come to the wedding, was his way of showing her that it was glad to see her so happy and that he would do whatever he needed to do, to celebrate and be there on her special day....he was telling her she was important to him, and he was prepared to make the effort for her.