The entry closely follows Photius, Lexicon pi575 Theodoridis s.v. penqera\ tw|= numfi/w| (attributed by Erbse to Aelius Dionysius: pi34); cf. Hesychius pi1398 and AB 228.32.
The headwords are used in Greek by the husband to designate his wife's mother and father; they are distinguished from e(kura/, e(kuro/s (epsilon 703, cf. kappa 1497), used by the wife for the mother and father of her husband. The entry is in error, however, over the use in Sophocles; and LSJ falls into the same error. See note 3 below.
The word gambro/s is a more general word, but this entry appears to give it its commonest meaning 'son-in-law'; cf. the citation from Sophocles. Chantraine has a useful analysis of this word: "In Homer 'son-in-law', but also in two instances (Iliad 5.474, 13.464) 'brother-in-law'; similar variability in usage in Attic-Ionian (cf. Herodotus 5.30 in contrast to 1.73, etc.); in the sense of brother-in-law it can mean a sister’s husband (Iliad, Hdt. 1.73) or a wife's brother; exceptionally it referred to a father-in-law (Euripides, Andromache 641, Hippolytus 635), indeed the word is used in a general sense for 'relatives'; cf. Aeschylus, Agamemnon 708, on which is based the passage in Pollux, 'gambroi, those from the house of the husband, such as his father and mother and all the others in the same way from the man’s extended family (ge/nos)' (3.31), a passage which claims that, the reverse of penqero/s, the word applies to the husband’s family" (Dict. Etym. 208-09). For LSJ see web address 1.
[1] Euripides, Alcmaeon at Psophis fr.7 Jouan/van Looy (= 72 Nauck, 102 Mette). The reading 'not giving you' (cf. Photius mh\ dw\s) is inappropriate to the context and is emended to pai=d’ e)kdou\s e) moi, 'having given me your daughter'. Jouan and van Looy posit two missing half-lines after 'old man' before 'having given'. In the context, Alcmaeon (OCD(4) s.v. Alcmaeon[1]) is wheedling a favour from his wife Arsinoe's father Phegeus (therefore his pentheros), for his new wife Callirrhoe, before being surprised in his deception and killed. The use of gambros is a deliberate prevarication, for a man has only one pentheros, and Alcmaeon now has Callirrhoe's father Acheloos as a second. For the story see OCD and Jouan and van Looy, 1e. Partie, p. 81ff., cf. Pausanias 8.24.8ff., Apollodorus 3.7.5.
[2] Euripides, Protesilaus fr.6 Jouan/van Looy (= 647 Nauck, 861 Mette). Protesilaus (OCD(4) s.v.; pi 2961), resurrected for three hours in response to his widow Laodamia's prayers, here addresses her father Acastus (misnamed both here and in Photius). Acastus regards his son-in-law's rights to Laodamia as wife and himself as father-in-law as terminated by death. A truly sophistic issue! For the story see Jouan and van Looy, 2e. partie, p. 567ff., esp. 580.
[3] Sophocles, Iphigeneia fr.305 Pearson/Lloyd-Jones and Radt (= 284 Nauck). The analysis here and in the other sources noted above, followed by LSJ, is based on a misunderstanding of the passage. Odysseus has been sent by Agamemnon on the mission to fetch his daughter Iphigeneia, for sacrifice at Aulis. Odysseus here dangles marriage to Achilles in front of her mother Clytemnestra, saying that the daughter will strike it lucky if she gets Peleus and the goddess Thetis as her parents-in-law. Either Odysseus in the play turned to address Iphigeneia or we should follow Lloyd-Jones, "But you, who are getting for your daughter a husband with great parents" (Loeb. edn., vol. 3, p.139). The word penqerw=n is particularly ominous, for its close resemblance to penqe/wn 'sorrows' reminds us of the tragic events that await them at Aulis.
LSJ cites one other passage under this sense, Euripides, Electra 1286. But there Orestes simply tells Pylades to take with him not only Electra as his new wife but also the labourer, to whose hut she has been exiled, and call him in name his penqero/s, i.e. pretend he is Electra's father. The LSJ entry at web address 2 should be corrected by removing §II entirely. See also web address 3.