In addition to what you have set up so far, you might want to add GV and/or another service, so that you can make or receive the calls using a mobile phone on one or both ends. When you have 5 to 10 timezones between parties, there is not a big window when both are at home and not sleeping, so having access from mobiles is very convenient. It should be possible to add this capability without sacrificing the privacy and security that you desire.
For example, with a Google Voice account on the European OBi, your mother-in-law could talk to her kids on their mobiles, at no cost except local airtime. Using the OBiON app, it may also be possible to avoid minutes usage.
Using a mobile on the European side is tougher, though if you provide some details (country, carrier, type of plan), there is likely a reasonably inexpensive solution, possibly by using the OBi's callback feature.