Ultrasound help

Hi ladies any help will be greatly appreciated, so basically this is my sisters early ultrasound below as you can see.. twins! now my question is the bottom sac has a heart beat 127Bpm and the top one they couldnt find a heart beat shes 6wks and 1 day however from what i can see and MY judement both sacs are growing at similar rates.. could it simply be too early to get the second heartbeat? maybe its measuring abit smaller than the other one? PS the bottom sac is the one with the heartbeat