Once is plenty! Upload it on one sister's card, then press + Add to more people and pick the others. The photo then shows on each sister's card, but there's still just one copy, so a change to it shows everywhere. For example, a photo of great-aunts Jean, Mary and Pat at the seaside goes on Jean's card first, then gets added to Mary's and Pat's.
Where to find it
A picture → + Add to more people
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