Nah. I don't even think Kopa was Simba's son. There's no way Zira could kill Sarabi, regardless of her age. Because Zira's an old hag, too. Sarabi's pretty big, one smack and it'd probably knock Zira right out.
I think she died of old age in her sleep, but that's just my theory. I do have my own Kopa theory here:
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