Main Difference
The main difference between Stepmother and Stepson is that the Stepmother is a female stepparent and Stepson is a male stepchild
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Stepmother
A stepmother is the current wife of one’s natural parent that is not one’s biological mother.
Wikipedia
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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”
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Stepmother (noun)
A viola, especially Viola tricolor, heartsease.
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Stepson (noun)
The son of one’s spouse, but not one’s own child.
Wiktionary