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Or helping your uncle Jack off a horse.

What's correct there, commas before and after Jack?

Depends very much on the uncle's name and occupation, he may well be an equine veterinarian named Steve.

That avoids the worst ambiguity but you still might be helping Uncle Jack murder a horse.

Phrased like that, I suppose that the horse would prefer the uncommaed sentence.

Or you might be helping your uncle Jack by murdering a horse.