N.B.
Make sure you tell him how you feel and that also with your hormones now going out of wack that it is really upsetting you. Since it has only been 4 months since you gave birth. I know it was around 3-4 months after my B&G twins that my emotions got out of wack, trying to get back to normal. It's also like a post partum thing too. So if he thinks your going crazy tell him it could be due to the hormones trying to regulate themselves again. But that is just how you feel right now.
Also since this is a lil sister. Did he have to take care of her a lot as a child? When you grow up having to watch over someone, it's hard to think of them as an adult.