My sister says she has traveler's diarrhea. are family members more likely to get the same thing?

Yes: Some cases of traveler's diarrhea are from ingested food. Other cases are spread from person to person. The best way to avoid getting infected is to practice very careful hand hygiene.

Minimally so: "travelers diarrhea" is common in areas of poor infrastructure, where your fook or drink may become contaminated by someone elses feces due to contaminated water or poor sanitation. If your sister's family members are traveling with her, then "yes", they are also at some risk. However, if good personal hygiene is used (good handwashing, etc), the contagious risk is very low.