I started training for a Tri in January this year at 6' 5" and the best part of 20 stone. Completed the Harlech Triathlon on April 13th this year 13 weeks later.
I think its great that you are encouraging your sister and with her swimming base she's miles ahead of most novice triathletes!
Repeating one bit of advice on this thread already, make sure her footware is sorted. It can be v.v.tough on the feet,shins and knees and I suffered alot until my Brookes Beasts!! took control. Havn't looked back since. (no weigh 16 stone something too!!)