I agree you have some great advice to read here.
My 2p is mainly to reiterate that them being siblings will not really make any difference now - they wont remember or recognise one another from their litter days.
So move forwards slowly as you would for introducing two stranger adult cats.
Lots of sent swapping (bedding blankets are great for swapping scent) and shared yummy mealtimes (either side of a door progressing to same room and eventually same corner in a room)
There is going to be a 'novelty factor' for your resident boy ... this will wear off and once ''play'' begins between them the level of aggression and frequency and manner of their rough play must be carefully observed for any fear or distress from either cat.
Its common for the new cat to accept rough housing and hide its fear when actually its confused still... ''Am I being attacked or what?'' fear and flight will be picked up by the other cat and this can trigger a real problem with constant stalking/fighting that isnt fun for them both and will really slow down your introduction progress.
At some point one will want to be dominant and most likely your resident boy ... its better if he can achieve (in his mind) the top cat spot with the minimum rough housing required... you can boost your boys territory ownership and ''position'' by adding a tree and praising him lots whenever he is up high on something.
I suggest that your new girls safe room be OUT OF BOUNDS for your boy ... enforced (calmly without drama) by you... make it clear to your boy that YOU are denying him ownership of your girls safe room and will step in to remove him/block access always... even during calm times its prolly best to establish in both cats that your boy isnt allowed to enter your girls safe room... it will serve to relax her and keep her safe room feeling safe until she doesnt need it anymore.
A common mistake to avoid is ending the introduction period too quickly when things appear to be all well and successfully completed.
Any upset go back a step in your process until peace is restored and then move forwards again gradually.
Please do update us on how its going... any piccies would be lovely too.