I'm guessing that "in general" this topic was already mentioned, but I wanted to discuss something more specific.

So, basically, my siblings is aware about my rich library of games. And they're willing to use it (from time to time, but still). Of course I've mentioned them something like "you know, I'm not going to refuse, but licence is on me, so in theory I shouldn't".But they're telling something like "don't be ridiculous" etc.

And to be honest, I didn't even told them that with conviction. However, whenever I've informed them about some freebies on GOG or even that I'm willing to gift them some games, they still had the same reply "you already have them on your account, why do I need to even create account on GOG". So yeah, even with my effort (and wallet) they're simply not willing to even make GOG account.

On the other hand - I can't say that I blame them, as this is the point of where "legal" stuff is crossing with something I would call "reasonable boarder"... Am I right? Or wrong? Do you have any thoughts about such (or similar) case(s)?

They will regret it when they are no longer living in the same household as you.

I'll refer to two slightly different -but related- support articles:
Can I enjoy my purchases both on my laptop and desktop computer at home?
Can I share games with others?

What you do with that is up to you.

Yes, I know about both those articles. I was rather going to ask other on forum about their thoughts (plus I gave my specific case as some single, detailed example from real life).

Let them use your games within reason. But moments like that are ones where you pull back and tell them to get a grip. It's a bit unfair for them to piggy back off of you and they need to learn that. It's like back in the day, you borrowed your friends CD/LP and taped them. Nothing wrong with that. BUT you still went out and bought the thing when the chance arose.

My though is that you should feel that you are somehow in control of the games. So:

1. Don't share your account details with them; only you should be able to download and install games (for them).

2. Don't share even the installers with them. Install the games yourself to their PCs, or make sure the installer is deleted from their PC after the game installation.

3. Restrict it to PCs in your household. So, if they live in the same household as you, then fine.

So the main idea in my eyes is that you should know who are playing the games you've purchased. If you give your account details or installers to someone else, then you've also given them the power to distribute the game further. If you merely install a game for them, then you can be relatively sure they are not distributing the game to their friends or the world.

All right, you didn't specify that you had consulted the support section, sorry about that. My main point was really to make your own judgement.

When GOG staff decide to chime in on these discussions, they generally recommend to treat the games as if they were distributed on physical media where you can install on multiple machines, but if someone else has the disc, you can't play.

I think there was a line in the TOS where they explained it quite nicely, treat games like you would physical copies. If you 'loan' them to a family member, don't play them at the same time.

Ultimately it's an ethical question. Do you feel comfortable sharing your games with your family even though GoG asks you not to? Then do it, no one can stop you. There might come a day however when they want to create their own account, because they want a game you don't have and then they might regret not having all these games in their own library...

Within my family, only one of my cousins is also a gamer and I freely share with him anything I bought on GOG. He doesn't have access to my account and it is me who downloads the games onto his PC.

I treat GOG games the same as if I had bought a disc version of them which I also always shared with him.

As long as you don't hand out GOG games to just about anyone or profit from reselling them in any way, it is completely fine IMHO. This issue is mostly dictated by every single person's sense of what is right and their conscience, which I think most people have at least some semblance of.

This is a strange one. How young are your siblings MartiusR? Do they all have direct access to your account?

Actually, that certainty depends heavily on how well versed the siblings and their friends are with Windows and its registry...
None of my GOG games has yet managed to hide its settings so deep that I would have been unable to get them work on another computer by just copying the installation folder and some registry keys.

Should the day come that I am not able to play a game from a copied folder on another computer, GOG will need to provide me instructions for digging up those missing settings so that I verify that the installer doesn't create some kind of a hardware ID which would eventually become an issue should the installer stop working on newer computers.

I say there are fair use cases.

After all, when I was a lad, we only bought one copy of Roller Coaster Tycoon, but installed it on our computers.

Dude, just tell them no. Your money, your games. You have no obligation to give them anything.