Usermanager most certianly does work work with SME 5.5 as I am using it on my setup.
By design, the Server Manager and User Manager are only accessable from behind your firewall. SME 5.5 provides a means for defining an external network that will be granted access to the server manager (although, I haven't used this feature). The catch is that you really need to be on a static connection to take advantage of this.
At one point, I remember reading on this forum of users configuring the server-manager to allow access outside their networs, but I recall the technique being somewhat advanced. What this really boils down to is: Yes, you will need to setup a VPN connection if you want mom to access the user manager. If you are running Win2000 of XP, no problem. You can setup a secondary network connection to your SME server in less than five minutes. I have such a connection on my laptop that I use to access my server-manager from home. After dialing in to my isp, I simply double click the connection to my SME network and preso, I'm connected.
The real question I'd be asking myself if I were you is why it is so important that your family members have access to the user-manager? Can't they fire you an email asking you to change their password for them? Unless you have a REALLY big family, this certainly won't result in my work for you.
Greg