the vaccine is not 100% protective so clearly your 12-year old was not adequately protected. yes it is possible that he may infect his 20 month old sister since she has probably just had one vaccine but i suspect she will be fine. the incubation period is about two-three weeks so you will find out at that time.
my 12-yr-old has chicken pox. vaccinated at 2 at 4. then refused booster shots. will he infect his 20-month old sister?