Depends on the variety, there are several different varieties of Hellebore and some are bigger than others - heights vary between 30cm-1.2 metres, and spread between 45-90 cm. More info here
https://www.rhs.org.uk/advice/profile?PID=113

If they are actually true Christmas roses, Helleborus niger they will not grow very tall - round about a foot max. But most Hellebores will be taller and make large clumps given time. Can you remember what time of year your sister's flower? Do you get a single white flower per stem quite low down usually in mid winter (like those on Chrstmas cards)
or something on a larger scale altogether? Ask your sister if you can't remember.