My question is whether I have to buy the add-ons such as a season pass twice on my ps4, since I have the psn credit in my brother's account and wonder whether the season pass or another one add on for all accounts is then available.
I would only buy once. If that's not enough, you can buy a season pass
Purchased content can be used by any user on a Ps4
Assuming the Ps4 is activated as the primary console