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1.
Growing up with a sibling of the same age or in the same grade that came about through the parent's remarriage.
My life forever changed the day I became a step-twin.

We don't know what our parents were thinking having step-twins.

Parent to school: Please don't put the step-twins in the same class next year. We don't think it'd be a very good idea at all.

Step-twins are the ultimate fraternal twins, as they not only came from two different eggs, but also two different mothers and two different fathers. Thus, step-twins almost never look alike.
by Harmony08 December 16, 2010
 
2.
Two babies born simultaneously to two different women, but with the same father.
We have the same birthday because we're steptwins, but our moms hate each other!
by kronq November 06, 2009