Hi Jennifer and thanks for sharing !
Being the new person into any existing relationship always can have it's trials. I think the important thing is to just acknowledge this up front and not let it be a distraction. And yes - it's FINE to just mention this to the existing team as a reminder of the difficult position a 3rd is in at first. Not in a whiney, demanding way - just an informational reminder.
If you think about it - it's no different than a new 3rd partner in an existing business. There's a pre-existing history, established routines and responsibilities. In a family there's sometimes children issues, financial issues, time constraints, a whole host of things already present.
Your role at first is to study it, learn about what's going on, what challenges there already are etc. Then see where you can fit it, be an enhancement. It's all about building something better for everyone.
But none of that happens overnight ! And it doesn't happen just because "we want it to". It's going to take some time, some effort, some experimentation, some tweaking. Don't take little conflicts and mis-steps personally. It's a learning experience for most everyone ! Make it a project to learn TOGETHER - and try to laugh at as many of the mis-steps as possible.
Eventually, as time progresses and everyone learns more about each other and the circumstances, it starts to gel, and the new 'history' gets constructed as a unit. Try to keep focused on that rather than trying to rewrite old history.
Hope that helps............