W-H has: Πίστει καὶ αὐτὴ Σάρρα δύναμιν εἰς καταβολὴν σπέρματος ἔλαβεν καὶ παρὰ καιρὸν ἡλικίας, ἐπεὶ πιστὸν ἡγήσατο τὸν ἐπαγγειλάμενον· for Hebrews 11:11.
Ralph Earle's Word Meanings in the NT contains a fine discussion on this verse. Earle informs us that most translations consider Sarah to be the subject of Heb 11:11, but the NIV regards Abraham as the subject: "And by faith even Sarah, who was past childbearing age, was enabled to bear children because she considered him faithful who had made the promise" (NIV).
Earle refers to F.F. Bruce's commentary on Hebrews (NICNT) which suggests that αὐτὴ Σάρρα be construed as a dative of accompaniment rather than a subject nominative. That is, instead of αὐτὴ Σάρρα, we should read αὐτῇ Σάρρᾳ (Nestle GNT).
Bruce Metzger lists two possibilities for this portion of Heb 11:11. Either Abraham can be taken as the subject of the passage or αὐτὴ Σάρρα στεῖρα should be understood as a dative of accompaniment (A Textual Commentary on the Greek New Testament, p. 602).
If we do, in fact, have a dative of accompaniment in Heb 11:11, it would then read:
"By faith he also, together with Sarah, received power to beget a child when he was past age, since he counted him faithful who promised."
According to BDAG, the dative of accompaniment is found in Thucydides X and Diod S. 20, 76, 1. See BDF 194.1.
Interestingly, John Chrysostom seems to have understood the text as a reference to Sarah being granted the power to conceive seed: he thus appears to construe Sarah as the subject like most modern translations do:
"By faith Sarah herself received power to conceive, even when she was past the age, since she considered him faithful who had promised" (ESV).