And at the same time you see Sara(as) the wife of Ibrahim(as) who was not infallible
Where do I see that? The whole talk before and after suggests she is an infallible.
1. Isaac and Jacob are mentioned.
2. She is grouped with such individuals.
3. The Quran says God chose the family of Abraham above the worlds.
4. She is mentioned to belong to a family that is blessed and who extensive mercy of God is upon.
What makes you think she is not chosen like Mariam? She gave birth in old age and was a vehicle of a miracle of God.
None of the virtues of sara(as) shows that she was infallible. A righteous woman can also achieve that. When Asiya the wife of firawn was declared as one of the best woman of all times that doesn't mean she was infallible. Even if I'm to assume that Sara(as) was infallible for arguments sake then that would mean that even Khadijah(ra) was also infallible. Because she was included among the best women of all times.
As for my beliefs that why don't I believe that Sara(as) was infallible is because of facts like these, which are not only in Sunni ahadeeth but Shia hadeeth as well.
We read in Shia book al-Khisal:
والمحنة في النفس حين جعل في المنجنيق وقذف به في النار، ثم المحنة في الولد حين أمر بذبح ابنه إسماعيل، ثم المحنة بالأهل حين خلص الله عز وجل حرمته من عزازة القبطي المذكور في هذه القصة ، ثم الصبر على سوء خلق سارة
Then it refers to a trial in his life when he was put in a catapult and thrown into the fire. Then it refers to a trial in his son when he was ordered to offer his son Ishmael. Then it refers to his trial through his family when the Honorable the Exalted God freed his wife from the hands of Azarat al-Qabti mentioned in history. T
hen it refers to his patience with the bad conduct of (his wife) Sarah. [al-Khisal, p. 308]
Similarly, We read in Shia book ilal al-shara’ie:
ان الأنبياء عليهم السلام كانت تسقط عنهم غلفهم مع سررهم يوم السابع فلما ولد لإبراهيم إسماعيل من هاجر عيرتها سارة بما تعير به الإماء، فقال: فبكت هاجر واشتد ذلك عليها، فلما رآها إسماعيل تبكي بكى لبكاءها
وسأله عن أول من خفض من النساء، فقال: هي هاجر أم إسماعيل خفضتها سارة لتخرج من يمينها، وسأله عن أول امرأة جرت ذيلها، فقال: هاجر لما هربت من سارة
Similarly in Shia book, Wasail al-Shia
وعنهم، عن أحمد بن محمد، عن علي بن الحكم، عن أبان الأحمر، عن محمد الواسطي قال: قال أبو عبد الله عليه السلام: ان إبراهيم شكى إلى الله تعالى ما يلقى من سوء خلق ساره فأوحى الله إليه إنما مثل المرأة مثل الضلع المعوج ان أقمته كسرته وان تركته استمتعت به اصبر عليها
We read in Shia book Tarjuma Maqbool, which is approved by twelve grand scholars of the sub continent.
In Tafseer Qummi, it is narrated from Imam Jafar Sadiq that Abraham was settled in a desert of Syria. Hajira(as) gave birth to Ismail. Sara(as) was highly saddened by it, that she had no child and Hajira gave birth to a male baby.
Sara would hurt Abraham alot regarding Hajira, and would aggrieve him. [Zamima Maqbool, p. 255]