How do I talk to my little girl about me not having a mum with out giving her the same fear of abandonment that I had growing up?

A bit of context.. my mother was abusive and had issues with drugs and alcohol. My dad fought really hard once they separated to get custody of us, he eventually got joint custody and while fighting for full custody she just disappeared one day and my dad was awarded full custody. I struggled with the fact I didn't have a mother and abandonment basically my whole life and this was nearly 30 years ago and my dad doesn't talk about it much as she was the love of his life and it still breaks his heart.Fast forward 30 years and I'm happy married with two step kids (M9 and F7) and two daughters of my own (4 & 2mths) my step kids are older and understand a little more the idea of separation, (we share joint custody and try really hard to support each other) so they sorta get it, but I'm really struggling with how to explain this to my four year old, I mean she's a smart kid and is putting together the idea of family based on our family and she keeps bringing up the question, where is my mummy, Iv done my best to scoot around it or simply say I don't have one but she continues to bring it up, especially since the birth of her baby sister, almost every day it's wheres your mummy whose tummy did you come from ect... She's too little to understand what happened but I also don't want to lie to her or imprint the wrong idea on her and then give her fears that I will leave her because my mummy left me.So looking to the internet for any thoughtful and helpful suggestions on how to talk to her about this or hear from anyone with similar experiences from either perspective.Thank you so much. via /r/Parenting https://ift.tt/2qbaCiI