I have to ask: do they know how to clean a tank? I am guessing that they do since you are expecting them to clean them, so maybe a different approach would be to setup a schedule for them. That way they have a visual reminder. I am not talking about marking a day on the calendar that they
must clean the tank. Just something that they can check off after cleaning the tank that week, or however often you want them to do it (I think you might be happy with a month
). Also, maybe you can
show them how to clean the tank efficiently. Showing them some "short-cuts" that you have found in cleaning five tanks might help. It sounds like they are finding the fish to be an annoying chore that they don't want to deal with anymore.
As for the feeding, it might help to remind them if they had their own food. Having the food in sight somewhere around the tank might help them to remember to feed them.
They are early teenagers and no one at that age is going to remember everything. Especially if it is something that they didn't gain their own interest in but found that they liked it because they saw their sister's (who started her own interest in it) tanks. It goes against what most other people here are telling you but I don't think threatening them with taking the tanks away is the best way to come across to them, especially to your sister that isn't even interested in the DP's anymore. Maybe you should ask if they actually still want the tanks and if not then maybe you should try and find a way to sell them.
P.S. I don't really see how grounding them is going to make them enjoy the fish any more. I realize they need to take responsibility in
choosing to use the tanks and fish, but making them dislike taking care of fish isn't going to solve anything.