ANSWER:

She was born out of wedlock and hence she is not your sister. However, you can’t marry her just to be safe but she is not your mahram. 

My father had an illegitimate child 30 years ago. We just found this out recently. Is this 30 year old lady a Mahram to me now?

Illegitimate children & Lost property, Mahram | November 24, 2020 8:47 am
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Brother I have 1 issue thats affecting my relationship with my mother. It makes me very uncomfortable/angry to be sitting in a family gathering with my sister and her husband [my brother in law] when my mother is not wearing hijab. I addressed this with my mother but she states he is like a son to her. I know theyre non-mahram to eachother and she never breastfeed him… her dean/iman is weak
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I have heard that when you lose something you should keep on saying “Innalillah..” and try to find it,is this authentic?