But for one reason or another, we're not as close now. Certainly not close enough to be 'best friends'.
We are, however, linked in a different, but very significant way.
Because in September 2011, Emily became my sister-in-law when I married her brother, Matt.
And now our daughters tell us that they are 'best friends'.
We don't see our family regularly. My Mum and sister, Louise, are in Devon and my other sister and brother are near Rotherham. Matt's family are in Ashford, over two hours drive from us. With our finances barely seeing us to the end of each month, we don't get to visit any of our loved ones as much as we would like.
And yet, the few visits we have had seem to have stuck in the minds of Lily and her cousin Ruby. Last week, when asked who her best friend was, Ruby answered 'Lily' and Lily claimed that Ruby is her 'favourite person'.
So it was wonderful when we got to have a family day out to Chessington on Saturday. It was touch and go in the week whether we would make it, our car broke down a week before and it took five days and £1000 to fix (ouch). I was incredibly grateful for our Merlin passes which meant that we could still afford the trip as we took a picnic and so spent very little on the day. Our passes also came with 'bring a friend' vouchers which meant that Nana, Uncle Jon and Aunty Emily only paid £10 each for entry (unfortunately, Grandad Paul came down with a tummy bug and couldn't make it).
It was yet another, amazing, memory-making day. The girls had a wonderful time, going on rides together, seeing the animals and watching the 'Pandamonium' show. Even the adults had a great time and managed a few rides for ourselves despite the queues.
I'm so excited for these cheeky monkeys. For all of the cousins (as I expect Ollie and Joel will be great friends too). I'm excited for their friendship and for all the happy memories to come. When they're older, I'll even buy them a corkscrew so they don't have to sieve the wine before they drink it!