I don't know if this will work, but it might be worth a try.
I assume on the marriage record for her second marriage that her married surname is given, correct?
Did any man with that surname die in the area in the previous year or two?
Can you find his marriage record and does the first name of the bride match your ancestor's first name? Track the couple forward. Did the wife survive the date of your ancestor's second marriage? If so then it isn't her. Repeat the steps for the next man on the list with that surname who died recently. Etc.
Obviously, this will work better if some of the names are unusual.
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Are there step children in the household in 1851? What do you know about them?
I don't remember the information on an 1841 English census. Can you find her on it? Can you tell if there are children in the home who are too old for that marriage?