This depends on a lot of diffrent things. Is this the first baby, what her build is like and numerous other things. Believe me there will be less time that she is not showing then when she is and after awile it can get very uncomfrotable so take advantage of the time you aren't. Good luck to your sister.
Depends on her size and if this is her first pregnancy. If she's a tiny little thing, than she may show earlier than someone who's a larger gal. She probably won't need maternity clothes (or even bigger size regular clothes) until she's about 4-5 months (16-20 weeks). She'll be able to tell that she's preggers, though, pretty soon. Her breasts will get larger, as will her waist. Oh, and nausea and fatigue should be kicking in pretty soon.
Everyone is correct. I am about 4 months along and just starting to show, am still in my regular clothes, but some are getting uncomfortable. This is my third but I am working out through it so far and I think that helps.
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!