I'd say it all comes down to how easy you want to make it for the siblings to use...
if they are happy with an FTP program that supports sftp (like filezilla etc) then you could just setup an ssh server (sudo apt-get install ssh) open port 22 on your firewall to the linux machine, create a user account for them (sudo adduser johnsmith then type in a good password twice) and they'll be able to up and download files from your PC (I'm assuming you've already got a static IP address / dynamic dns setup) (NB there's more you'd want to do in terms of securing this but that would get you a working system.)
If they can't handle something as "complicated" as ftp then you probably want to go the route of a VPN and samba... If you're not happy with the command line then that ebox thing is probably the way to go.
There is a ready to run ISO which you probably want to go for... my install onto a ubuntu vm just failed (probably because I didn't bother reading the manual
) and I'm just off home but I'll post back if I get a chance to look at ebox in more detail.
Though considering the amount of administration 3-4 users are going to need (IE, probably very little) it would be much quicker to use standard command line stuff.