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.
Step-great-grandchildren.
Reba has step- grandchildren
Your step-son is the child of your spouse and another person, and is of no true genealogical relationship to you. So your step-son's children would also be of no true genealogical relationship to you either. However, they would commonly be called your step-grandchildren and you would be their step-grandfather or step-grandmother.
Your niece is the daughter of your brother or sister. That relationship is not ended by divorce. If your brother married a woman who already had a daughter (from an earlier marriage), then she would be your "step-niece", and this relationship would, technically, be ended by a later divorce.
They're step-grandchildren.
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
It's a matter of how strong your relationship was with your step-daughter. It all depends from how you feel about this all. But just answer yourself a question, what will happen with your step-daughters children if you won't look after them.
Your grandchildren and your grandniece are second cousins to each other..
Reba has step- grandchildren