I retired a few years ago. My pension was being paid under the 50% Participant and Spouse form until my wife died a couple of months ago. Can I have my widowed sister replace her as my beneficiary?

No. You cannot do that under this Plan. If your spouse dies before you, your benefit will pop up to the unreduced amount that would have been payable if you were not married when you retired, but you may not name a new beneficiary.