Legally changing last name after marriage

Hello! I recently got married and just received my marriage certificate. My sister is also in the process of changing her last name legally, and ran into a bit of a tricky sitaution with the application process. We need to know our mother’s legal last name for the application. The problem is, she is divorced from my Dad (that we are aware of) and we have a strained relationship with her. She will probably not tell us whether she legally changed her name or if it is still her maiden name. She is also mentally ill, so I’m not sure she will even remember this specific information.

I’m looking for advice on whether we can figure out this information without asking my mother as this will trigger a great deal of stress/emotional turmoil.

Thanks for any help!