Grandparents spying on my mom’s emails and transactions. What info do we need to hire an investigator and what would they be able to do about it?

My grandparents are narcissists. They have narcissistic personality disorder and want their way all the time, want all attention on them, and want to have total control of anyone they can. They have always been very nosy and had a weird way of wedging themselves into our plans or just knowing we were doing something or going somewhere. We always blew it off as them just always wanting to know what we were doing. If I had a drs appointment, half an hour before she would call while my mom was on the way taking me to the appointment and she would ask if we were doing anything or how I was. If we planned a trip she would call and ask if we had trips planned. Things like this would go on for years. If my mom was thinking about buying a car, they would start talking about car prices. It never really crossed our minds because how could be spying on us? But just about a month and a half ago, we had planned a trip to (said city in said state). They live in the next state over. This was going to be where we were going for Easter and my grandparents always stay with their side of the family for Easter. (We are pretty separated from them as they are all kinda messed up). Literally right after my parents received confirmation for the plane tickets my grandparents called my mom and said “hey we are going to (said City in said state) why don’t you come and see us?” We immediately said oh cool we’re staying home and we canceled our trip. My mom got a really weird feeling because of how immediate the response was. My mom then wondered if maybe they had access to our emails, and I helped her and say that yes, we received plane ticket confirmation through her email. And all my mom’s transactions, my doctors appoints, and my brothers school stuff all went through my moms emails. It all started to come together. My grandparents must be seeing and reading our emails. To try to further confirm this my mom deleted all her messages and started emailing my dad, me, and my older siblings really “juicy” things. All lies but something that would catch my grandparents attention. Like my sister would email my mom saying she’s pregnant, or my dad would email my mom saying we’re getting sued. Things like that. And like clockwork, my grandmother called my mom asking her about us. Asking if we were all ok or if anything’s going on. These last few weeks she and my grandfather have gotten more and more nosy. Always trying to get a hold of us or start weird conversations. There’s definitely something going on and my parents are wondering if we should hire a private investigator. If we do what proof do we need and what would an investigator be able to do for us. Thanks. TLDR grandparents likely have been spying on my moms transactions and family stuff through her emails and we want to know what proof is needed for a private investigation and how that would help us