I think the squirt bottle and the time outs are a good idea. Kittens will be kittens and eventually she will get the hint that hissing means "leave me alone!" And eventually either the older cat will REALLY lay down the law or he/she will get used to the attacks and just put up with them.

Welcome to PT!

Kitten 645, we posted about the same time and had the same idea about kittens will be kittens.