Is a sibling with chicken pox dangerous to my 7 month old baby?

Depends: They may have both been exposed to the same source at the same time & break out differently within the usual range of 10-21 days from exposure. If not, the 2nd began exposure from a day before the spots appear.Simply being in a room together with a case for 1/2 hour is enough to pass it. Infants seldom have as many spots or complications as average kids. It is worth monitoring & a call to your doc