It depends on what you mean by parents and sister organizations. Are these organizations on the same network with you or are they connecting through VPN? You'd want to start looking at software firewalls which might be located on the server hosting the FTP services. Likely that is not the case if your local computers are connecting with no problems.
Then you need to check your router/firewall that would filter all of your incoming/outgoing connections. This is where the VPN question comes in. Some routers allow specific rules for VPN users that are not applied to local users, etc.
If they are coming in over the internet using the FTP protocol, then you'd need to check your main router/firewall again here to be sure FTP is allowed over the internet (but I don't recommend you allow port 21 over the internet).
Let's start with that. Let me know what you find and we'll go from there.
Also, do you have a log of the error messages or anything you are getting?
