Is this normal?

About a week ago, our cat Bella had kittens. So far, we've kepther in the bath room, with the door closed so that the other cats wouldn't bother her, though we leave it open when we're up there, and the other cats usually go in and sniff around (Or try and eat her food. Or use her litterbox.).
One of our cats, Molly, who is Bella's sister, would usually go in and poke at the kittens, so we've been keeping her out. But a few nights ago, we watched her to see what she would do. To our suprise, she jumped into the box they were in, curled up with the the kittens (Bella was out, stretching her legs), and began to purr and lick them and act like Bella did with them.

Ever since then, she has been determend to be with them; she stands outside the door, scratched it, and if it's open, runs as fasts as she can to the box and the kittens.
We really hate kicking her out fo the bathroom, but my mom is afraid thta Bella might not come back to them when she leaves if Molly is always there.

Anyway, the reason I'm pothing this is because I was wondering why in the world she's doing this, and if she'll stop.
Also, if it's okay to let her stay in there; it brakes my heart to keep her out; she just seems to love them so much ...

So, yea; any ideas?

MY brother had this problem with Spaz. She would still Shadows Kittens and run away with them. onve 2 were missing and he found spaz in a closet with them.

She's jealous, wants kitties of her own. Lots of animals are this way - you should see some of the chimps Jane Goodall's worked with, and watched them even steal other chimps' babies (though I'm not suggesting Molly would do that). But the chimps also babysit for each other on purpose, and nurse each other's babies if necessary, and all the females fuss over new ones.