Your post says that your son is now afraid of lightning and now has a pest of a baby sister -- how does that translate into hating you?
I would let your son talk a lot about his feelings about the thunder storm. I would try to think of questions he might be thinking but can't put into words. I would encourage play with his toys where there are thunderstorms in what he is playing and the toys can react all kinds of ways. Any and all ways that he can work THROUGH his feelings and get them out, over and done with. Sometimes telling him what to think (they can happen anywhere) helps you, the adult, but isn't helping from a child's viewpoint.
If your son (and you) are having trouble adjusting to the new little one, a great book is "Siblings without Rivalries". Also, you can have him help you attend to little sister, if he's not resentful of that. You can also talk to him about him when he was a baby. He probably would like to hear "when you were this little, you would..." stories.
Grandparents houses are magical just because they're not your own. :) I know with my son, I am a given and everything else is neater or funner. But when all is said and done, I am the cornerstone. Be there for him, focus on keeping your relationship strong and you will always be love in human form.