Rental income is subject to income tax after deducting all expenses related to the rental property. The sublet aspect does not affect the taxable impact of receiving the rental payment.
There is no way to answer your second question without having access to the specific information on your tax return. If the rental income is such that when added to your existing taxable income you are pushed to a higher tax bracket, then yes. If the rental income is such that you don't encroach your next tax bracket, then your tax rate will not change, but your total income taxes will increase by the amount of the net rental income received during the tax year.
Hope this is helpful!
Answers are simply - net income from property rental is taxable income to the holder of the lease and recipient of rental payments.
