Can I add additional names to our joint bank account for future reference, and is there a limit to how many names can be on the accounts?

My mother-in-law has 4 other people on her bank account with her. I am the POA (only) but her three sons (I'm married to #3) are all on her account with her. When she dies my job is done, but her sons are basically 'co-owners' of her monies and can do what they want with it.