(I’ve moved this to projects so you can get individualized responses based on your specific setup. 
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Hopefully people who actually have something like this working will chime in. You’re right that you will have to have separate smartthings accounts, not just two locations on one smartthings account, or the Alexa control won’t work correctly. If you want administrator access to your mother’s smartthings account, you’ll need to set up a new email address and have her invite that one instead of inviting the email address you have associated with your own smart things hub. Otherwise, just use smarttiles to get on/off Device control of the account for that the other person’s hub.
So each person has their own smartthings hub, their own Echo, their own Amazon account, their own email address. They use the SmartThings mobile app on their own phone to have administrator access to their own SmartThings account. They use smart tiles on their phone to give them On/off access to the devices at the other person’s home.
www.smarttiles.click
Presence
So that leaves the presence issue. You going to need two different ways to establish presence for the same person, so that each smart things hub will see them.
Obviously you could use two different devices, namely an arrival sensor for one hub and the phone for the other, but I think you can do better than that.
The following is just theoretical, because I haven’t tried it myself, but it might be possible to use the SmartThings mobile app As the presence indicator for that person’s main account, and use Life360 on the same phone (using The secondary email account) to establish presence for the other person’s hub. Life360 is a third-party service and has an official SmartThings integration which is quite popular.
https://support.smartthings.com/hc/en-us/articles/202334244-Connecting-Life360-with-SmartThings
I’m just not 100% sure that that will work, or if just the fact that there is a life 360 app on the same phone as the SmartThings mobile app will in some way tie them together.
If it doesn’t work, there are three alternatives that I do think will work, they’re just more effort to set up.
First, you could set up an IFTTT account for each person, using a third email address. Link that account to the other person’s smart things hub. (I know you can have more than one IFTTT account linked to the same smartthings location, we have three at our house.) use IFTTT presence to turn on a virtual presence sensor at the other person’s house.
Or use connecting to the other person’s Wi-Fi turn on a virtual presence sensor at the other person’s house.
You could also use an IBeacon at each person’s house and have the phone turn on a virtual presence sensor for that house’s hub.
All of these methods get a little complicated. But they are detailed in the presence FAQ with links to threads were people have done them. And they’re all based on the same basic idea: each person will have two different ways of defining presence, one for each hub. How many email addresses or devices that will require ends up depending on the specific method you choose.
So I’m pretty sure it can be done, but there may be some trial and error in testing which method works best.