Family members like Wives and childrens of prophet Muhammad are of ahlul byt. Then who is the first woman of ahlul byt (who accepted Islam)?
Sadly the original post has been changed a few times and I wonder if the answer of @Noor would be the same by now.
I think all Muslims do agree that the first lady among ahl al-Bayt which accepted Islam is our prophets () wife Khadija bint Khuwaylid() one may differ on whom it was among men (even among the Sunni's there are three plausible options), maybe among whom was the very first person ... etc. as some evidences seem to contradict others.
Here just two quotes from siyar a'lam an-Nubala' سير أعلام النبلاء of a-Dhahabi when quoting Khadija () -my own translation take it carefully-:
قال عز الدين أبو الحسن ابن الأثير : خديجة أول خلق الله أسلم بإجماع المسلمين ، لم يقدمها رجل ولا امرأة .
'Izz ad-Dyn abu al-Hassan ibn al-Atheer said: Khadija was the first of Allah's creation to accept Islam, by consensus of the Muslims, and neither any man nor woman came before her.
This consensus sounds rather doubtful if we read:
وقال الزهري ، وقتادة ، وموسى بن عقبة ، وابن إسحاق ، والواقدي ، وسعيد بن يحيى الأموي ، وغيرهم : أول من آمن بالله ورسوله : خديجة ، وأبو بكر ، وعلي .
One of the narrations which appears in both sahih's which may show that Khadija was the first believer among the ladies of ahl al-Bayt is narrated on the authority of 'Aisha () - I will just post one of the shorter versions, note that among the longer versions there might be some discussion due to differences in the content in what the students of ibn Shihab az-Zuhri delivered-:
He (the Holy Prophet) came to Khadija an his heart was trembling. The rest of the hadith has been narrated like one transmitted by Yunus and Ma'mar, but the first part is not mentioned, i. e. the first thing with which was started the revelation to the Prophet was the true vision. And these words like those transmitted by Yunus are mentioned thus: By Allah, Allah would never humiliate you. And there is also a mention of the words of Khadija: O son of my uncle! Listen to the son of your brother.
(Sahih Muslim. The longer versions from sahih al-Bukhari 1, 2 and 3 and in sahih Muslim)