I think it is very normal, moreso that you have an extra member of the family who is a few months old rather than the 2yo "thing". I think Natalie was just soooooo stroppy that she had a sibling for what felt like absolutely ages. But oh, it really does get better! And it happens gradually so's you don't really notice. They just happen to manage better together and they adjust and all that. We really had some shocking days when P was 4-6 months old: most days in fact. I definitely remember not liking her very much some days Then the shockers decreased as time went by, and now the shockers are a rarity.
Keep at it! You are doing wonderfully.