There are reports of Leonardo being present at high profile meetings between Vito and his closest guys. I think a sentence of such is stated in The Sixth Family book, but I'm not sure. I don't find it very surprising or have much doubt if he is involved in the organization in some criminal aspect. It wouldn't be the first time a lawyer turned mafiosi. Him not being killed when his brother and grandfather was is a good sign he isn't involved, but at the same time it doesn't mean much.