You might also consider other relationships. My great-grandfather's parents died leaving their children orphans. My grandfather, who was only 4, and one older sister went live with someone who is, I believe, their step-uncle or step-grandfather. Two other children went to live with an older half-brother. People then seem to have been really good about stepping up when the need was required to take in orphaned children. Often when the census was taken, the people responding to the enumerator didn't see the need to give correct surnames of children who may or may not have been their own biological children. This is why it's so wonderful when we can find other records to support any theories we might have. Best wishes on solving your mystery!