You are a "great uncle" to your step-brother's grandchildren. You might even be called a "step great uncle" but most people don't bother specifying step-relationships that distant.
You are not related as you do not share a common ancestor.
You brother is your grandchildren's great uncle or granduncle.Your grandchildren are your brother's grandnieces and grandnephews
The relationship term is usually hyphenated "great-grandchildren" (grandchildren's kids).
they would be second cousins
No, they are not stepbrothers. They are best friends though.
Your grandchildren and your grandniece are second cousins to each other..
your great grandchildren
aunt
Great aunt
Your grandchildren and your niece are first cousins, once removed, to each other.
The name of the relationship between your grandchildren and your sibling's grandchildren is called being "second cousins." The genetic relationship between two families becomes smaller and smaller through each generation, but no matter how weak that relationship is, it's still counted as a blood relationship. Your grandchildren and your sibling's grandchildren share a pair of great grandparents.
No. Dale Jr. and Jeff Gordon are not stepbrothers.