You shouldn't have any problems over the weekend, if you are keeping an eye on your electrical use.
Question....how old is the "old" battery? If it's more than a few years, you may have an un-equal load on the pair of them, which may (or may not) cause a problem.
Since it doesn't sound as if you'll be using the inverter (you didn't mention A/C or microwave use), and only using the batteries when you are about to go to sleep, you should be fine.
With 3 people taking showers, I think I'd be more worried about water use than the battery use:)