As per your description your brother Seems to have developed infection at the Site of graft harvesting in the leg. Which is very unfortunate since he has been through such a major surgery and this must have added to your anxiety
a great deal.
As far as your question goes i would say that infection at the site of surgery could be due to many reasons and yes one of them is hospital aquired infection but not necessarily in all the cases.
Despite taking all aseptic precautions still a minor percentage of the cases develop infection at the site of surgery.
I hope he recovers soon with antibiotics and rest.
Wishing him a speedy recovery