Ultimately I think you want to define what you want your overarching Brand Experience to be. That starts with a purpose for your collective, and the promise that you want to make to your customers, on which you need to deliver across all the different sub-brands that you have, and all the touchpoints within those brands.
At the high level, from there I’d define your overarching positioning and strategy – what is it that you want to achieve together as a collective in the next 2-5 years, and how will you get there? What makes you different from others in the market? This is what will tie all of these organizations together.
One thing I also think is important to understand > do you all target the same customer? If the target customer base is wildly different you may want to think about how you go to market, as your messaging may get confusing.
I hope this helps~