Hi and welcome to PT. This is normal behavior so please don't worry. He's probably just trying to get used to her scent and he may also be trying to wash her bottom. My furkids do this to each other all the time.

I always thought that only dogs would sniff each other bottoms but cats do this a lot too. I'm glad to hear that your older cat likes his new baby sister. I'd love to see some pictures of them.