Wiktionary
n. A stepfather of one's spouse.
Wikipedia
A stepfather-in-law is a stepfather of one's spouse.
The children of the stepfather-in-law would be the person's stepbrothers-in-law and stepsisters-in-law.
Category:Family
Usage examples of "stepfather-in-law".
This old geezer had been starving to death for twenty years on the wages my stepfather-in-law give him, and the rest of us didn't make no holler when his name was read off for a small chunk, especially as he didn't have no teeth to enjoy it with.
On the contrary they tell me that when the big news hit Niles the old bird had a stroke that he didn't never recover from, and though my wife and Katie hung round the bedside day after day in the hopes he would tell how much he was going to leave he was keeping his fiscal secrets for Oliver Lodge or somebody, and it wasn't till we seen the will that we knew we wouldn't have to work no more, which is pretty fair consolation even for the loss of a stepfather-in-law that ran a perfume mill.