My son is extremely close to his grandad (my dad), they love each other to bits.
My dad is not my birth father, he came into my life when I was 8 and later adopted me after he married my mum when I was 12.
I met my birth father twice in my life, once as a toddler then when I was 12 before he agreed to the adoption. I am now 43 and no idea if he is even still alive.
My Dad is very much my DAD and I love him every bit as much as my mum. He cared for me, nurtured me, made me a part of his family etc etc
I know from my mum that since my son was born that he has worried about when the whole 'not real grandad' thing might rear its head and that worry grew (for him) with the increasing bond that they developed together.
I've been matter of fact about it. When the situation has arisen, I've spoken about 'mine and granny's life before we met grandad'. For example I've shown my son where I was brought up and he knows that only me and his granny lived there. When the inevitable question arose 'where was grandad' I explained that we hadn't met yet, but I always wanted a dad and didn't granny pick a great one for me!
As he got a bit older I spoke about where I was born (a different city) and that granny had been married to a man called 'john' at that time but they didn't want to be married anymore and so that's why me and granny came back (to home town).
"So you used to have another daddy" my son said. "Yes I did, but now I have your grandad".
My son hasn't ask anymore questions yet (he's 7, I know they might still come). I know he understands, but it doesn't matter a jot to him. His grandad is his grandad.
My advice would be to just drip feed the truth to your girls in the most natural way as situations and conversations develop. No secrets, no big revelations. Families can be diverse, it's not necessarily a bit deal if you don't make it one x