I am 14 weeks and am showing...ive been showing since about 12-13 weeks but it depends on the build of the woman and what kind of shape she's in when she gets pregnant. I am only 5'3'' so there arent many places for this baby to go except out. I wasnt in the best shape when i got pregnant, i weighed about 120 so I started to show earlier. If you abs are super toned and in shape you'll show later and it'll look tighter. I kind of got the chubby look for a few weeks and now it's finally all concintrating into my stomach which Im super happy for
It also depends on if this is her first child or not. Women who have had children before tend to show sooner because your body responds and remembers what it has to do and starts making room a lot sooner. Hope this helps, congrats to your sister!