Most plan administration that identifies a "contingent" beneficiary does so only in case the "primary" beneficiary (usually, as specified by the employee) is not surviving at the time benefits commence to a(ny) beneficiary. Often, this does not extend to identify a "secondary" beneficiary as you describe. Often, the primary beneficiary is permitted to select his/her own secondary beneficiary (if applicable).
In your case, perhaps neither scenario has been anticipated. The Plan Administrator (or administrative committee, etc) may need to create an administrative interpretation. If in doubt, don't hestitate to contact the ERISA legal counsel who drafted the plan document.
(In 30+ years, I've never seen any plan that paid a pre-retirement J&S to a sister.)