If you do, please share!
We are a newly blended family with myself, my 10yo DD and my DH who is her step-dad. We are thinking about writing something like this ourselves. Right now we are feeling inspired by beginning with the set of "expectations" DD's school has, which are about safety, respect, and participation (but written in a much more poetic way). Of course, a mission statement would not really be about rules but something deeper. But somehow this set of expectations feels like a starting point. I would love to hear how other families are going about this!
Not sure if this is what you are after but we have a set of "family rules" which the kids came up with at one of our family meetings, with a bit of guidance from us. They work to guide our interactions with each other and with out friends/family/community. I have been frankly amazed at the impact they have had on our parenting and discipline, and how we so often turn to them to help us sort out what we should do next if we are stumped with how to handle something. I can absolutely see how they help the kids be their best selves in all kinds of situations and how they encourage me to maintain the standard I want in my parenting. I will often see kids standing in front of them (they are on our fridge) and using them to work through a problem or reminding each other to use them when we are out and about.
Always be kind
Always be helpful
Listen to each other
Use your words to solve problems
Respect ourselves and each other (this one has evolved slightly as the kids have gotten older)