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.
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.