What federal tax and penalties applies to my wife's 401k after her death? Am I obligated to roll it over into mine?

You are NOT obligated to roll the 401K into your 401K or into a rollover IRA.

You CAN have the money distributed, but you will pay normal income taxes (federal and sate) without the 10% Early Withdrawal Penalty.

However, doing the rollover is probably your best option tax-wise.