Your wife's affidavit is NOT less important. If her income was below the filing threshold during any of the years, she just checks that box, and hand writes which years, if not all. Your petitioner is always the primary sponsor. The joint sponsor is also needed, but both forms and sets of supporting documents must be correct and sufficient.
If your wife is currently employed then you state her current income. Here's the formula. Take the gross income from a full pay period and multiply it by the number of pay periods of that length in a full year. So 1k a week is 52k a year, etc. That's her current income. Tax returns are about the past. No need to state and document liquid assets in this case.