My brothers sister passed away yesterday...

Ok, so my dad met this women many years ago and they were engaged till she got pregnant with my brother. All of a sudden she wanted nothing to do with me or my dad, I don't know why. I have only met my brother 1 time when he was a baby. I have seen him around town here and there though, he is not about 10-11. He has no idea me or my father exist. She forced my dad to give up his parental rights and she remarried and had a little girl that had a disease that sadly killed her yesterday. All I can think about is my poor brother. I want to comfort him and be there for him so badly! I also want to send his mother a condolence card. I recently just lost a baby sister to trisomy 13 and its been so hard on me and my family. I can't imagine how hard this is for her. I do not want to tell my brother who I am without her permission, but I would just like to tell her how sorry I am and maybe see if there is a possibility I can meet my brother in the future. It kills me to not know my own brother. I never did anything to her, it was all my father (I do t think he did anything to her either, I just think she didn't want a baby and was pissed when she got pregnant). Do you think it would be ok for me to send her a card?... This is so hard for me right now...

***UPDATE*** I wouldn't be wanting to tell my brother that I am his sister right now or that his father isn't his real father. I'm just wanting to send a condolences card saying I'm sorry for her loss. Nothing more, nothing less.