I hope I'm not about to offend or upset anyone,but I need some advice before doing something daft.
My "cousin" is my half sister, my mum gave birth to her 10.5,years before I was born. My mums sister and husband adopted my half sister.
My mums died several years who, whenever asked about my cousin/half sister I was always told "I 'll tell you when you're older" - unfortunately that day never came 
My cousin/half sister knows she is adopted and and my mum is her birth mother (but my aunt (now dead) is in her eyes her mum). I gave no problem with this. However, I am curious as to my half sisters father- he knew my mum and if he us still alive I'd like to talk to him about my mum (my own dad died in1984) - so would it be crass to aka my half sister if she has details about her birth after (I suspect the answer is yes) - any advice from anyone gratefull y received (my half sister and I don't have a close relationship - I have intermittent contact with her two daughters)
Sorry it's so long and rambly - just feeling down and very alone right now 