Ahsan Allahu ilaykum: My mother prays with a khimar thats white and thin/cotton based. If she stands under a ceiling light and if im in front of her — i can see skin color as she wears half sleeves inside. Is it valid to pray with this? And what is upon her if she did hundreds/thousands of prayers?

This could be waswas as it could be truly thin and transparent or just ocd. Imam Ibn Baz said a woman is all awrah in prayer except her face, and if her clothing is thin and does not cover, her prayer is invalid; a garment through which the skin colour shows is not a covering. But we have to make sure this is not just ocd.