I hope the op goes well for him!
My understanding is that healing is usually longer for people with diabetes, so expect a slow recovery, and he may well need extra care himself during that time.
During his post operative recovery period he will be assessed, and support offered, as appropriate. However, you may need to get other teams involved to arrange specific care for your mother, since the post op assessments will focus on your brother, not your mother.
I would suggest that the earlier you start this, the better. And please be aware that you if you feel you are not being listened to, or need someone to speak for you, that you can request an Advocate to attend any meetings.
A friend of mine recently went through this with her father, who was hospitalised for an infection, but his mental health (Alzheimers) took a massive step downwards, due to the change of scene and stress. It became very clear that the hospital just had one priority - freeing the bed for the next patient - and she had a great deal of difficulty in achieving a result that was actually in her father's best interest! Obviously, that is a very different situation from your brother and your mother, but it is worth remembering that each professional and team that you deal with is working within the priorities of their team, not necessarily with the overall benefit to the patient in mind.