I'd put that picture under "Outings/Vacations" or "Family: _________(name of your sister)"
or "Road Trips"
Here are a few category suggestions:
travel & vacations
heritage
weddings
parties & celebrations
holidays
religious events
births & birthdays
places
friends
meals & recipes
work
military
family & relatives
reunions
gatherings
sports
school
fun & play
colours
season
date;time;year
It might help to browse through your pictures first to get an idea of categories that are meaningful to you. For pictures that do not easily fit a theme, sort by other criteria such as individual: son, daughter, mom, etc. For some categories you may want to further sort with added criteria like person: birthday - son, daughter; heritage - mom, dad and so forth.
Or event: Holidays - Christmas, July 4.
Or time: Holidays - 2000, 2001. Or combine multiple criteria: Holidays - Christmas: 2000, 2001.
Think of it like a file structure on your computer. The level of detail for further sorting is up to you.i recommend that you only sub-divide a category that has lots of pictures.
When you're all done sorting, look at what's left. Now it's decision time. To keep or discard? It's often difficult to trash the left over items, but don't fret over it. For pictures that you're not sure about tossing, put them in their own container and label it with the date. In a year, see what items are still there and if you've used them or not. If you haven't, then consider discarding them.
Have fun organizing your photos!:))