I am sure all of the Big-4 and various generalist consultancies will offer "sustainability audits" somewhere in their marketing materials, but in reality it isn't an area where they are credible or have sufficiently large teams to offer internships (unless, as you say, you are related to one of the partners).This is an area where the 'engineering' consultancies have an edge on the 'management' consultancies due to their expertise in environmental management, planning and impact assessment, HSSE, etc.Have a look at Atkins, HTSPE, Mott MacDonald, etc. There are also economic development organisations e.g. Adam Smith International which don't always like to describe themselves as consultancies, due to having charitable and political links. Check out the Economist for names of other firms.