Main Difference
The main difference between Stepmother and Stepson is that the Stepmother is a female stepparent and Stepson is a male stepchild
Stepmother
A stepmother is the current wife of one’s natural parent that is not one’s biological mother.
Wikipedia
Stepmother (noun)
The wife of one’s biological father, other than one’s biological mother.
“bonus mother|q1=euphemistic|co-mother|q2=same-sex and polygamous families”
“stepfather”
“stepparent”
Stepmother (noun)
A viola, especially Viola tricolor, heartsease.
Stepson (noun)
The son of one’s spouse, but not one’s own child.
Wiktionary