All your Mil can do is her best. Same for you, if Mil won’t let you help, there is not much you can do. If accounts or other assets surface at a later stage, they can be dealt with then.
It is often only when it comes to probate, that proper searches can be done and long forgotten about accounts / assets frequently turn up.
The HSE has previously said that 25% of Fair Deal applicants who since passed away had under declared their assets, some anyway are honest mistakes rather than deliberate non disclosure.