When my dad was young, he had child with a one-night stand...
He took care of my half sister for years and when she was 16, she decided that she wanted to live with her mother and not my dad. My dad married my mom around that time and we haven't heard from my half sister since, (unless she was calling for money.)...

Recently, my mom was on FB and happened to read a message that my half sister sent her... Basically the message said, "I'm 44 years old now. I am a mother and a grandmother and I haven't seen my dad since I was 16. I don't understand how a parent could not speak to their own child for that long! I am also a step-parent, and as a step-parent, I would always encourage my man to stay active in his children's lives..."

Basically, she blamed my mom for not encouraging my dad to keep in contact with her. I can understand if she's mad that my dad wasn't active in her life, but she never reached out in 30-some odd years...

I want to reply to her... She's basically blaming my mom for her bad relationship with my dad instead of blaming herself. She mentioned in her message that my dad was a "great dad", yet she still seems upset for something that was partially in her control?

Should I say anything to her? Or should I just stay out of it?
I feel like I should say something, after all, she's trying to put the blame on my mom instead of on herself...