Hello, I'm Dr. Deb.
I recently came online and see that your question about hasn't been answered. I'm so sorry that you've had to wait for a response, but if you still need assistance, I'd like to help if I can.
I can understand why you might be concerned about the older kitten's decreased interest in the canned food you're putting out but if she's gaining weight and otherwise acting like a normal kitten should, I don't know if I'd be too worried about it. It's possible that she just doesn't like the taste of the brand you're feeding any more or know that's she's bigger, she may have decided that she doesn't really like canned food that much.
This is a phenomenon that I've encountered with cats in the past: They have very decided preferences about what they like and what they don't in terms of flavor and consistency of food (canned versus dry).
I wouldn't necessarily let the younger one finish off what she doesn't want since she's going to become overweight if she's taking in more calories than she needs. If these were my cats, I'd only put out 1/2 the amount of canned food for the older one as you have been doing since that's all she wants to eat.
Every vet has a different protocol as to when they like to spay cats but usually it's around 6 months of age so you have a bit of time before this procedure is done.
I hope this helps although, again, my profuse apologies for the delayed reply.. Deb