How long does it usually take for one child to pass chicken pox to a sibling?

Chicken pox: The incubation period for chickenpox is 14-21 days with the majority of susceptible children beginning to manifest fever and vesicular ( pox -pink base with clear blister on top) lesions by day 10. The lesions appear in "crops" over 4-5 days. A child is considered contagious up to 48 hours prior to onset of symptoms and up to 24 hours after last 'crop" of lesions has occured and crusted into scabs.

Not very long.: All that is needed is droplets from a sneeze or cough, or contact with the clothing, bed linens, or oozing blisters of an infected person; so a few minutes of contact is all that is necessary. After that, there is an incubation period of about 2 weeks before the sibling gets the rash.

Depends: They may have both been exposed to the same source at the same time & break out differentlywithin the usual range of 10-21 days from exposure. If not, the 2nd began exposure from a day before the spots appear & would break out from 10-21 days later. Simply being in a room together with a case for 1/2 hour is enough to pass it.