Yes they are in laws
Your nephew's wife is your Niece-in-Law, whether she is caring for a child from a previous marriage or not. Few people would care to be that precise, however, so most will probably just call her your Niece.
Your sister-in-law is either the wife of your brother or the sister of your spouse. The children of your brother's wife are your nieces and nephews, since they are also the children of your brother. In some cases they may be your sister-in-law's children from a previous marriage. Then they are not related to you at all. However, if they are very young, it may be convenient to call them your nieces and nephews anyway. The children of your spouse's sister are always your nieces and nephews.
Most people just say in-laws. Technically, I think they would be nieces and nephews, but only by marriage.
The nephew of a wife and the nephew of her husband are not considered to be related to each other. However, both are nephews of the wife and the husband, and the wife and husband are aunt and uncle to both nephews.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
I think the young marriage I would consider is like 20-25. That's what a young marriage would be.
If it is a boy then it is your nephew. If it is a girl then it is your niece.
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.
Not in the typical understanding of the word. Generally, a relative would be someone who is related by blood. A spouse would, specifically, be "related by marriage".
You are not related to all of your brother-in-law's family, but you are related to some of them. You are related to your brother-in-law. You are related to his wife, your sister. If he and your sister have children, you are related to them - they would be your nieces and nephews. But you are not related to your brother-in-law's siblings, cousins, parents, aunts, uncles, grandparents, etc.
Yes, there is nothing that would prevent it.
You would be his Great Uncle and he would be your Great Nephew.