The fear is legitimate and the concern comes from a real place, but the honest answer is that worrying about what is happening inside is the part of this you have the least control over.
Drugs and alcohol are available inside correctional facilities. That is not a secret and it is not going to change. Contraband finds its way in through a variety of channels that facilities work constantly to close and never fully succeed in closing. If your brother wants to use inside, the opportunity will exist. If he does not want to use, that same opportunity will be easier to resist than it would be on the outside where triggers, routines, and social circles are all working against him.
That last point is worth sitting with. Prison, as miserable as it is, removes a lot of the environmental factors that feed addiction. The people he used with, the places he went, the stress patterns and coping mechanisms that led him to the bottle, none of those exist inside in the same form. Some people get sober in prison not because the program was perfect but because the circumstances forced a break that the outside world never allowed.
The harder truth is the one the original answer states plainly. If you could not get him sober on the outside, you cannot get him sober from the outside while he is inside. Addiction does not respond to someone else wanting recovery on the person's behalf. It only responds when the person themselves decides they are done.
What you can do is be honest with him in your letters and calls about what you see and what you hope for, without making every conversation about his sobriety. Stay connected. Encourage him to engage with whatever treatment programming the facility offers. And take care of yourself in the meantime, because loving an addict is its own exhausting work regardless of which side of those walls they are on.