It is true that your photos are being saved on your NAS. So the downloaders you send invitations to need a way to connect to the NAS to get them.
By itself this does not leave your home systems "open to hackers". Just like putting your return address on letters you mail to your friends does not open your home to burglars.
You should have good security on your home network anyway (even if you do not use photos II).
Did you forward yoh-dah's response to your brother? viewtopic.php?f=145&t=62638&p=351188#p351188 It is quite possible that he did not realize what login screen he was looking at.
If that does not address his concerns, maybe he can join the forum, and post his security concerns personally?
BTW, you should probably follow up on your security concerns thread, as that one is only about your topic.