Nursing on each other?!?

We have two 3-month old kittens, both female. They were with their mom until they were about 9 weeks or so. Dash tries to nurse off of her sister, Astrix. She'll wait until Astrix is on her side, and then she'll move in and start sucking on her sister's nipples and kneeding her stomach and purring. Obviously no milk comes out. We are so confused! She eats solid food just fine! Can any harm come of this, especially for poor Astrix, who doesn't seem to mind the nursing but looks awfully confused? Will Dash outgrow it? Should we get her some formula and bottle-feed her? Everything else she does is completely normal. Help!

One of my cats was doing this to a litter mate when I picked her out as a kitten. I didn't do anything special, she is now almost three and perfectly normal.

I discourage this with bitter apple. Just dab a bit on the kitten's stomach and the littermate will leave her alone. You can also order a www.snugglekittie.com as a substitute mom