Scott
It depends on whether she is receiving Contribution Based ESA or Income Related. If the former then your earnings will not affect her ESA, if the latter then your earnings will reduce her ESA £ for £.
Also, if you are working more than 24 hours a week, her EWSA(IR) payments will cease.
I would put your joint information into one of the online benefit calculators to see what they say.
www.gov.uk/benefits-calculators Gordon