It will be taxed at 28% because it is added to your total joint income and taxed at your top tax bracket.
But filing separately will not save you anything. She may only pay 10% on her individual income; but more of your income will be taxed at 28% because of the lower thresholds on the Married Filing Separately ( MFS) brackets.
You can use this tool to do trial returns both ways. https://turbotax.intuit.com/tax-tools/calculators/taxcaster/?s=1
