Oklahoma does recognize the stepping into another's shoes doctrine whether they be your family or not.You can't use deadly force under the SDA to "rescue" someone you don't know. You can however use deadly force if you have reasonable fear of death or great bodily harm, say, if they tried to attack you as you aided the elderly couple. It's a fine line and not one you want to approach lightly.
That said you better know exactly whose shoes you are stepping into before you act.
Walk up on a lady with a guy pointing a gun at her and you shoot the guy:
1) If he was the bad guy you be OK
2) If he was the good guy and he had disarmed the bad girl you gonna get a new wife named bubba and enjoy her company in the OK State lockup for a few years.
The story of the gas station owner shooting federal DEA agents noted in another post is a good case in point and it was used in the CCW class I took 10 years ago.
Stepping into another's shoes is legit but you had better know for sure whose shoes you are stepping into before you do it. IMO not doing it is probably the most practical response. In reality the values of many here would not permit them not to step into another's shoes in many if not most situations.