Doesn’t sound your MIL is able to protect herself emotionally from what she is going through with only SIL there. They can look into her money and where it has gone. Should probably alert the broker as well in case MIL shows up with some notarized something changing the current arrangement. Hope everything works out for her and for you also.
but I commend your DH for handling his mom so gently yet being firm to the SIL through her. Relaying the events to his brother was a very good idea emotionally as well as for a witness, etc. Must be reassuring to have his support and know that he knows what is going on. Hope that this is resolved quickly and that your DMIL is able to be free of it all.