I am saying that the Qur'an is in Arabic, and that it is full of Arabic terms, so the Arabs, to whom it was revealed, could understand it.
When they read that Musa (as)'s sister said to Pharoah's people "I'll find an Ahlul Bayt", they understood it as "I'll find some women of a house".
Some women of a house.. which house?
If there was a man instead of Nabi Musa's (as) sister then it would make sense that he is referring to his wife by saying Ahlul Bayt. The arabic word "Ahl" means "People" in English language e.g. Ahlul Kahf (People of the Cave), Ahlul Qurra (People of the Town), Ahlul Medina (People of the City), Ahlul Kitab (People of the Book), etc.
Example 2:
وَحَرَّمْنَا عَلَيْهِ الْمَرَاضِعَ مِن قَبْلُ فَقَالَتْ هَلْ أَدُلُّكُمْ عَلَى أَهْلِ بَيْتٍ يَكْفُلُونَهُ لَكُمْ وَهُمْ لَهُ نَاصِحُونَ
And we ordained that he refused suck at first, until (His sister came up and) said: “Shall I point out to you the people of a house(ahli bayt) that will nourish and bring him up for you and be sincerely attached to him?”.(28:12)
Even in the verse above the mother of Moses(as) is termed as ahlebayt, not because she was mother of a prophet, but because she was the wife of Imran. And some people like shia scholar aqa pooya mahdi whose tafseer is refered by popular shia website al-islam.org have completely misunderstood this verse, they think that here mother of Moses(as) was termed ahlebayt because she was mother of a prophet, no this is wrong understanding because, the statements which the sister of Moses(as) made , were to the soldiers of firawn , if the sister of Moses(as) was addressing the lady with term ahlebayt because she was mother of Moses(as), then the people there would surely would have questioned her that who is the mother of his child and why did she put the baby in the river and then everything would have been messed up.
Similar is said by a different shia tafseer:
(The agents of Pharaoh became happy by that statement and began going with her toward that lady. Moses’ sister, who showed herself as an unacquainted person and a stranger, informed the mother of the matter.) (The Light of The Holy Qur’an by Ayatullah Sayyid Kamal Faghih Imani and A Group of Muslim Scholars, under explanation of verse 20:40)
And if people use their common sense then they can understand this verse easily that, in no way the sister of Moses(as) was signifying any relation between the child and mother of Moses(as), had she done so, she would have been in deep trouble. But infact she addressed mother of Moses(as) with “ahlebayt” because of being the wife of a person(Imran).
Its is possible that after reading convincing response to the illogical arguments raised by them, the shia might try to take a U-turn with their theories , they might say that in this verse a complete household is being addressed, not just a single lady. So to such arguments we answer from the Quran itself . Because the best way is to explain the Quran is through the Quran. For, what the Quran alludes to at one place is explained at the other, and what it says in brief on one occasion is elaborated upon at the other.
Quran says: See how We repeat the verses that they may understand.” (6:65)
“And certainly We have repeated for mankind in this Quran, every kind of similitude, but the majority of mankind do not consent to aught but denying.” (17:89)
Its clear from quran that there was no need for a complete household, But just a single woman who could nurse the child. So why would sister of Moses(as) refer to a complete household? Moreover another verse of quran is more clear to solve the confusion that was it a complete household addressed by sister of moses(as) or just a single lady with the term “ahlebayt” ?
Your sister went to them and said, “May I show you “someone” who will nurse this child?”(sarwar shia translator, 20:40)
Even explained similarly by shia commentators:She told the men of Pharaoh whether she introduced a “woman” to them who was able to nurse the baby. The verse continues saying: (“…’Shall I direct you to one who will nurse him?’ …”) Maybe, she added that this “woman” had a pure milk so that she was sure that the child would accept it. (The Light of The Holy Qur’an by Ayatullah Sayyid Kamal Faghih Imani and A Group of Muslim Scholars, under explanation of verse 20:40) )
From popular Shia website, Al-Islam.org, which contains the authentic Shia Tafseer of Pooya/M.A. Ali. :When it was picked by Firawn’s family and they seemed to love the child, she appeared before them and promised to bring a good “wet–nurse” for the child.(pooya ali, tafseer al islam.org 20:40)
Similar is said in another shia tafseer i.e Tafseer namuna vol 7, page 359
Even sunni commentators explain the same:
she then said, “Shall I show you “someone” who will take care of him?”. Her offer was accepted and so she brought [them] his “mother” and he took to her breasts.(tafseer jalalayn 20:40)
She meant , “Shall I guide you to “someone” who can nurse him for you for a fee” So she took him and they went with her to his real mother.(tafseer ibn katheer, 20:40)
Quran itself answers such misunderstandings , where it clears that sister of moses(as) referred to single women “someone”… the Qur’an is its own best commentary . As we proceed with the study of the Book, we find how true this is. A careful comparison and collation of passages from the Qur’an removes many difficulties.
And moreover if for an instance for sake of argument we agree that it was a complete household addressed, even then no one can deny that fact that, mother of Moses(as) was included in it, and not because of being the mother of the child, as we explained why, but because being the wife of Imran.
https://youpuncturedtheark.wordpress.com/2010/10/06/usage-of-word-ahlahlebayt-in-quran/In the case of Hadhrat Ibrahim (a.s) and Hadhrat Sarah (s.a) their son Hadhrat Ishaq (a.s) and grandson Hadhrat Yaqub (a.s) are also included in term "Ahlul Bayt." Hadhrat Ishaq (a.s) and Hadhrat Yaqub (a.s) are also part of Ahlul Bayt of Ibrahim (a.s).
Example 5:
قَالُواْ أَتَعْجَبِينَ مِنْ أَمْرِ اللّهِ رَحْمَتُ اللّهِ وَبَرَكَاتُهُ عَلَيْكُمْ أَهْلَ الْبَيْتِ إِنَّهُ حَمِيدٌ مَّجِيدٌ
She said: O wonder! shall I bear a son when I am an extremely old woman and this my husband an extremely old man? Most surely this is a wonderful thing. They said: Do you wonder at Allah’s bidding? The mercy of Allah and His blessings are on you, O people of the house(ahlebayt), surely He is Praised, Glorious.(11:72-73)
In this verse too, wife of hz ibrahim(as) is addressed as ahlebayt, And she was addressed because of being wife of hz ibrahim(as), As explained by quran itself in another chapter:
فَرَاغَ إِلَىٰ أَهْلِهِ فَجَاءَ بِعِجْلٍ سَمِينٍ
He went quietly to his wife(ahlihi) and returned to his guests with a fat, roasted calf.(51:26, sarwar shia translator)) .