First I want to clarify that I have question related with buying from GOG not the game itself.

I share this account with my siblings, one of them preordered this (he bought 49.99 euro base version), and I want to know if I want to buy higher tier preorder, will I buy it at asking price or at cheaper 20 euros difference between those two versions?

I've never been in this situation, so unfortunately, I don't know the answer to this. When I look at higher-tier versions of games I own in the store, the price of the higher-tier version remains unchanged, so I'm assuming you'll have to pay full price for the higher-tier version. Some games also feature "tier upgrade" DLC you can buy to add the content of the higher-tier version of a game to your lower-tier version. It's possible that Pathfinder 2 will have such an upgrade option at launch, but I wouldn't count on it.

However, if you want to pre-order a higher tier version of the game for an account which already has a lower-tier version pre-ordered, I'd suggest contacting GOG Support with a refund request. This will give you back your 49.99 Euros, and you can then buy the higher-tier version using that money + the additional 20 Euros.