Kittens drinking milk?

[staralise]

My girl kitten (Cleo) is about 9 weeks+ old. We got her at about 5 1/2 weeks old and she gets along great with our male kitten (Stormy) who is about 12 weeks+.

Cleo was eating and drinking fine when we got her, she gets clean water and milk daily (which I change a few times) + dry and wet food which I make sure is always available. In the last week she has been trying to drink from Stormy - what would make her suddenly do this?

It confuses me because she was fine drinking milk from her bowl, but now shes trying to drink out of Stormy. Why isn't she stopping when she doesnt get milk? Shes constantly doing it even once I watch her drink milk from her bowl.
Sorry I didn't make it clearer. She is trying to suck from his nipples.

The two kittens are from totally different litters.

I do believe that she was taken off her mother too early, but she was fine up until just now.