How do I tell my children they have sisters?

I live in Australia but am from the UK I have 2 children with my partner (4 and 2 years old) and am currently pregnant with our third. I also have two children from a previous relationship (21 & 18) who unfortunately live back in the UK. My older children from my previous relationship left to return to the UK when my other children were very young and because of that the younger ones do not remember them. Both me and my partner know that this is due to us not keeping them in their memory but when my children left, for various reasons (mainly to do with their father back in the UK meddling) our relationship wasn't good and we didn't talk for a while. Things have been good for sometime now between us but I have been struggling as to how to tell my other kids that they actually have two other sisters. I know the moment I would tell them my 4 year old especially being older would want to meet them but I don't know when or if ever this will happen as we can't afford to fly them over here and they have made it plain they never want to ever live here. I don't want to confuse her but don't want to leave it and do a Jerry Springer on them and tell them when there 18! It is mainly my 4 year old that we are concerned about as I know our 2 year old won't understand. How do I explain to our 4 year old that she has two other sisters who live miles away? are we doing the right thing doing it now or should we wait til later? I hate that I don't have any pictures of my other two out or that when we do talk about them we talk about them in code we feel awful doing it but because we haven't been sure how to tell her about them we thought it best to do it this way. If I could change how we handled things in the past I would but I can't so how do I deal with it now?