I did this with the younger half-siblings of two of the sims in my current save. I played a single mom who had twin boys with another sim I created and moved into a different neighborhood. It never ended up working out with the parents, and the dad ended up marrying and having children (also twins, but is one male, one is female) with another woman. When the dad and his wife died, MCCC moved the twins into two different households. When I learned this, I used manage households to move them in with their half-brothers, even though, by this point, their mother had died of old age and they were already raising their other two younger siblings. They were an interesting family to play.