Yes, I understand. Even tho your mom abandoned you and you were raised by your step mom, you still want to make a connection with your 'birth' mom, you want to experience the connection.
You will experience the visceral connection and hopefully something good that you can hang on to, but you will also experience the abandonment.
My philosophy is that it is healthier to get it all out on the table, and experience a relationship for what it is. She did give birth to you and I'm sure there was a real connection and affection and then she really chose her addiction over you and your sisters. That has to hurt.
However, it is better to experience all of that, rather than suppress it; avoiding the pain also annihilates the good stuff.
It sounds like your sisters are having a harder time with the pain. I can understand that, too.
And...its not surprising that she is sounding like a child...she's troubled.