1. I would not say it. If it's my husband's sister's child, it's my niece or nephew, not my "niece-in-law." And if it the person who has married niece, I'd just say "my niece's husband."
2. I would just say "My brother-in-law." If that doesn't seem precise, you could say "my sister-in-law's husband." One time I did say "My husband's brother-in-law" and the person I was talking to said "Uh