Digital Forensics of Personal Emails

I need a digital forensics expert to analyze a large number of emails over the years involving my mother ("M") and brother ("B") in my personal email accounts on Gmail and Outlook. I am virtually certain that B, who has occasional access to M's account, has been monitoring M's emails most likely by having set M's Gmail account to auto forward emails received (possibly also sent) to another Gmail which M used long ago but has not used in over a decade. B has access to the old account and most likely set up the said forwarding so as to create plausible deniability. Unfortunately, I cannot confront M about this as B would immediately find out and likely manage to disable the forwarding rule by accessing M's account with some excuse. So I want to know whether any of the email activity provides any objective evidence or strong indication of B's monitoring. Alternatively, I want to find a way to obtain that evidence.
Note, that some of the evidence would likely come from what I never noticed before but now realize has been B's deliberate use of the old email address in some of the emails (that is to include both M's old and current addresses).