I would take the "easy way out" here and tell them you are unable to take that much time off of your job. You don't have to tell them that you didn't actually ask your boss about it - just explain as you have here that there's nobody to cover your position, and week long vacations are the most anybody takes.
This avoids hurting their feelings by saying you don't really want to spend two weeks with them, and instead they can redirect their disappointment onto your boss (aka somebody they can't argue with). You say your visits are usually a week long, so it's not like a sudden change.
I think that directly telling your family will inevitably lead to hurt feelings. There's really no nice way to say "I don't like spending long amounts of time with you". Unless you can propose a solution, I wouldn't bring it up. For example, perhaps you could stay at a hotel, or if they promise to respect your privacy when you close your door, you'd be willing to stay longer. Otherwise - and if nothing can change your mind about staying longer - there is no point in telling them the reason.
Regardless, make sure that they do know you still care about seeing them and look forward to your time with them. This will lessen the sting and keep everyone on positive terms.
Personal experience: I recently switched jobs and have very little vacation time accrued so far; I have personal reasons for avoiding being at home too much this holiday season. Technically there are ways I could get around the lack of vacation time (advances, overtime, etc.) but I have just told my family sorry, only x hours vacation so I have limited days I can visit. So far no hurt feelings since it's framed as "not that I don't want to see you, but I can't" - which is partially true in both statements as I do want to see them, just... not for long amounts of time at the moment.