Hello i still don't know how she agreed to accept Islam and married the Prophet after her family was killed please ANSWER ME so i can tell others 
AsalamuAlaikum,
This is what my next upcoming article will be about.
There are many reasons:
She fell in love with the Prophet SAW before meeting him:
نَزَلَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ خَيْبَرَ وَصَفِيَّةُ عَرُوسٌ فِي مَجَاسِدِهَا، فَرَأَتْ فِي الْمَنَامِ كَأَنَّ الشَّمْسَ نَزَلَتْ حَتَّى وَقَعَتْ عَلَى صَدْرِهَا، فَقَصَّتْ ذَلِكَ عَلَى زَوْجِهَا، فَقَالَ: وَاللَّهِ مَا تَمَنَّيْنَ إِلا هَذَا الْمَلِكَ الَّذِي نَزَلَ بنا، فَفَتَحَهَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَضَرَبَ عُنُقَ زَوْجِهَا صَبْرًا، وَتَعَرِّضَ مِنْ هُنَاكَ مِنْ فِتْيَةِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لِيَتَزَوَّجَهَا
When Allah’s Messenger (pbuh) reached Khyber while Safiya was a bride at her place. She saw in her dream that a sun came into her lap. She mentioned it before her husband. He said; ‘By Allah you do not wish for anyone except the King who has come upon us.’ Allah’s Messenger conquered Khyber and her husband was killed. From that time (of dream) she loved that the Prophet (pbuh) may marry her.
[Tabarani Kabeer, Hadith 19667]
Also, the symbolic fall of the sun onto Safiyah's chest had its own indications. For she had a yearning, from the depth of her heart full of faith, for the person of the Messenger of Allah, Muhammad.
[Muhammad 'Ali Qutb, (2008) Women Around The Messenger, International Islamic Publishing House, p101]
During her childhood her parents and uncles were worried about the Prophet Muhammad SAW because they knew he was a prophet:
"When the Prophet emigrated to Medeenah, he stopped on his way in a place called Quba'. When the Jews heard of his arrival they hurried to see him to make sure that he was the epected prophet and they started comparing him to what was written in their books about the last prophet expected to rise... Safiyah said: ... Then I heard my uncle, Abu Yasir saying to my father: "Is he the one?" My father said: "Yes, he is." My uncle said: "Are you sure of that? Are you sure that he is the Seal of the Prophets?" My father said: "Yes, he is the last Prophet."...
[Faridah Mas'ood Debas, The Wives of the Prophet Muhammad, International Islamic Publishing House, pp.123-4]
So as a young child she knew he was a Prophet.
Aswell her husband use to hit her:
When the Prophet entered Safiyyah's presence, he notice a bruise on her face; he asked her about it and she said: " I saw in a dream that a moon had fallen into my room and I informed my husband, Kinanah about it and he said: "This means naught except that you desire Al-Hijaz and Muhammad!" Then he struck me a blow to my face and this is the mark left by it."
[Darussalam, (2004) The Honorable Wives of the Prophet, Darussalam, p.115]
And as we know the Prophet SAW never hit his wives and taught good treatment of women:
The Messenger of Allah () said: "The best of you is the best to his wives, and I am the best of you to my wives.
[At-Tirmidhi Book 49, Hadith 4269 Grade: Sahih (Darussalam)]
The Prophet said: "The best of you is the one who is best to his wife, and I am the best of you to my wives."
[Sunan Ibn Majah Book 9, Hadith 2053 Grade: Hasan]
The Messenger of Allaah said: ‘Be kind to women.’”
[Narrated by al-Bukhaari, 3153; Muslim, 1468]
“Aisha reported that Allah's Messenger, may Allah bless him, never beat anyone with his hand, neither a woman nor a servant, but only, in the case when he had been fighting in the cause of Allah …” (Sahih Muslim, Hadith 4296)
It was narrated that 'Aishah said:
"The Messenger of Allah never beat any of his servants, or wives, and his hand never hit anything."
[Sunan Ibn Majah Book 9, Hadith 2060 Muslim. Grade: Sahih (Darussalam)]
There are so many reasons and in shaa Allah in my article I will detail them.
