The child came with mother on the F1, and the stepfather petitioned for child after marriage. Child was petitioned as stepchild of US citizen. Marriage and conditional green card was issued prior to child turning 18.
Conditional and 10 year GC was issued because the marriage was deemed to be legitimate and there were no questions about the F1 visa. The mother ran into issues because the school that issued her the F1 years ago was convicted of immigration fraud years later.