It's a milestone in your relationship to be considering building toward marriage/kids. Congrats!
Take it easy though and don't plan to make all these changes all back to to back. Each one separately is change enough! Give time and space for each step along the path. You can look up those kinds of life changes on the stress scale.
Don't be piling stress up on yourself if you can spread it out some.
Could focus on living together for now with an eye to near future.
If he wants to be with her and tolerate her personality issues, that is up to him- but her negative behaviors not interfering in our relationship are a requirement for me sticking around, and Joe knows it.
How is this boundary
impacted by you and Joe living together? Because I do not see how you can be "free of impact" when you live in the same flat -- you will witness Sue phone calls, drive bys, etc. much more in yo' face than now living separately. How does living together impact your ability to leave?
At this time I see two ways to approach living together:
You can baby step into living with him. Maintain your place and pay rent... but pack a bag, empty the fridge and go live with him. When she comes, go back to your place. "Maintain separate bedrooms in separate homes" approach. Then you can see the fallout without having to deal in being on a lease/breaking lease issues.
You can go more directly into living with him. Give up the current places and get a place together with enough bedrooms to keep "separate bedrooms in the same home." Here you WOULD be on the new lease, and maybe you don't want to sign and move in until you have saved enough in a separate account to bail and get a place of your own again? Your Plan B money?
Because last thing you need is to move in with him, have her behave badly, you hit limit and break up with him... and here you are. Still living with him because you are stuck on the lease for a while having to witness wacky you want to be free of.
I think "try before you buy" is one way to go -- could not create more entanglements or commitments like (marriage) and (children) until you see how Sue's behavior plays out and how Joe's character holds up just with (living together). Could try on a small commitment of sharing a lease first before taking on bigger ones like raising a child. If you have to baby step to sharing a lease, could baby step it.
I don't really think it's a matter of anything I do- I think her own issues will cause some lashing out no matter what- and of course, Joe is going to be sitting there second guessing his choice of moving forward with me because of how negative her behavior will become. I don't even want to imagine her reaction to marriage and babies...
You may not want
to imagine it but you have to in order to consider your options and plan for reasonable near future -- like a 5 year plan. If nothing else to save money for doing these things.
If his character is such that you wonder if he's going to be second guessing living with you? Do you also wonder if he is going to second guess marriage to you, children with you?
Take time to sort all that out with him. Then take some more time living together to SEE FOR YOURSELF how good he is at keeping promises and maintaining healthy boundaries with Sue when the pressure cranks up.
Have you though this far out? You marry him, you have babies and she wigs and you break up and divorce him because of your boundary. How cool are you with your child going off with Joe and Sue witnessing their dynamic you do not like or approve of? Because he shares custody? Is that a total deal breaker for you? Or something to put in the prenup/post nup re: child custody agreements?
And what's your involvement with his son gonna be?
Talk about these kinds of things with Joe so he's clear on your boundaries and you are clear on his. Make sure you are both on the same page.
And the usual stuff applies here too -- sharing chores, money, religion, children's education, extended familiy, etc. Maybe you guys want to take marriage classes at your county extension office, place of worship, or online.
Talk that stuff over. I am a fan of loooong Engagements. Could engage, and do the prep work and all the big talks. If after doing the work you come to find you are NOT actually compatible for bigger commitments? That's was an excellent Engagement! Doesn't lead to a wedding, but a good Engagement doesn't have to. A good Engagement is about serious preparation and serious consideration.