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Hmmm. I don't know about the actual political cartoon, but I tried Googling simply "Please spare our mother" and got this here, which includes a joke with the same premise. Incidentally, the trope that you linked to, Aren't You Going to Ravish Me? already includes the same basic joke in the Jokes section:
"During one of the Roman civil wars, a soldier bursts into a villa and finds two young women and their elderly nurse. "Mercy!" cry the young women, "do with us as you will, you brute, but respect our beloved nurse's grey hairs!" "Hush, children," chides the nurse, putting down her spindle; "war is war!""
I remember coming across a late 18th/early 19th century political cartoon, possibly around the time of the Napoleonic wars. I wanna say it was by James Gillray, or at least a similar artist from the same time period.
It featured two soldiers, two young woman, and an old woman (their mother or grandmother), with dialogue something to the effect of:
I was trying to find it as an example for Aren't You Going to Ravish Me?, but no matter what combination of words I searched for, I couldn't find it.
Edited by Admiralakbar1