LXX Deut 25:5

Ἐὰν δὲ κατοικῶσιν ἀδελφοὶ ἐπὶ τὸ αὐτό, καὶ ἀποθάνῃ εἷς αὐτῶν, σπέρμα δὲ μὴ ἦν αὐτῷ, οὐκ ἔσται ἡ γυνὴ τοῦ τεθνηκότος ἔξω ἀνδρὶ μὴ ἐγγίζοντι· ὁ ἀδελφὸς τοῦ ἀνδρὸς αὐτῆς εἰσελεύσεται πρὸς αὐτὴν καὶ λήμψεται αὐτὴν ἑαυτῷ γυναῖκα καὶ συνοικήσει αὐτῇ.

When brothers dwell together and one of them dies without a son, the widow must not marry outside the family. Her husband’s brother is to take her as his wife and fulfill the duty of a brother-in-law for her.