My sister says she has ptosis. Are family members more likely to get the same thing?

Unlikely. Most ptosis is individually acquired and not inherited. Your sister's ophthalmologist can evaluate her and tell you why she has it. You are unlikely to have the same history.
Not usually. However, a tendency toward dermatochalasis, which is an age reated droopy lid with puffy upper lids does tend to run in families and hasomething to do with the upper lid structures, an inherited tendency. Family members will have similar facial structures, so as we age and sag, the likelyhood increases.