Is it normal for professors to not have families or not spend time with their families over the holidays, due to pressure of work? My sister is a professor.  I was visiting her over Thanksgiving break and my father (who is also a professor) told me not to hang out with her because she had a grant proposal due.  Is this normal for professors or is it just my family?

This happens almost every time we have a family reunion, and my father never wants me to talk to my sister because she always has some sort of deadline.  It gives me serious doubts about going into academia.
The extent to which academics lead deadline-driven lives varies tremendously.  It differs between fields (even otherwise similar fields; for example, theoretical computer science tends to have far more paper submission deadlines than pure mathematics does), between types of institutions (such as research universities vs. liberal arts colleges), between career paths (such as running a research group that requires substantial ongoing funding vs. doing individual research on a theoretical topic), etc.  There's also a large factor of personal taste: some people thrive on deadlines, while others try hard to avoid them.

It's also worth noting that deadlines often repeat in cycles.  For example, yearly conferences typically have submission deadlines at the same time of year, as do certain funding opportunities.  Which conferences or funding opportunities are most important/relevant vary between people, so two professors could have completely different schedules of deadlines throughout the year.  It's possible that your visits with your sister happen to line up with a pattern of deadlines that matter to her.  That would be unfortunate, but not necessarily representative of the rest of her time.  In my experience deadlines are not particularly likely to coincide with holidays.

If you are curious you could always ask your sister (or, for that matter, your father).  For example, "I noticed that you often have deadlines around Thanksgiving.  That sounds pretty stressful - is this typical for how you spend your time?  What's your job like on a day to day basis?"