No - Your nephew is not considered a related party. You will be able to take the loss up to $3,000 on your 2019 return. Any remaining loss will be carried forward to your 2020 return.
The IRS defines a related party in part as:
Members of your family. This includes only your brothers and sisters, half-brothers and half-sisters, spouse, ancestors (parents, grandparents, etc.), and lineal descendants (children, grandchildren, etc.) [IRS Pub. 17 - Your Federal Income Tax, p.104]
