I know it does not totally answer your request, but you totally can achieve that with Crashplan; and for free.
Simply buy a NAS or equivalent, place it at your parents and install Crashplan on it.
It is not as perfect as a totally untied system like you want, but it is free and offsite :).
I am a happy customer of Crashplan for a while but other than that I have no ties there :).
Yeah, I think it is close but not quite. It seems that you can back up to a friend's trusted computer, but it has to be turned on. Plus I don't necessarily want to trust them in that way.
I know it does not totally answer your request, but you totally can achieve that with Crashplan; and for free. Simply buy a NAS or equivalent, place it at your parents and install Crashplan on it.
It is not as perfect as a totally untied system like you want, but it is free and offsite :).
I am a happy customer of Crashplan for a while but other than that I have no ties there :).
https://www.crashplan.com/en-us/