Still confused and unsure and it's hurting everyone

[Before you can make things better, you have to stop making them worse...]

Is it not YOUR home as well? Do you not have a right to invite YOUR family into YOUR home?

---

I would say that this actually strengthens your case for having your sister over to your house for the event. If it were on neutral ground at a rented venue, then your wife could make a bunch of noise about having a "right" on who to invite and who not to invite. However, in YOUR home, she does not have the right to tell you to keep your family away. Period.

Thanks. Helps a lot to have confirmation that that position is a reasonable one to take.

This behavior is directly attempting to alienate you from your family of origin...without any reason other than irrational dislike of your sister.

That's the thing: my wife is adamant that it's nothing to do with dislike and everything to do with my sister mistreating her. It's just that the "mistreatment" most definitely doesn't deserve that name, and in any case is mostly just healthy person reacting in shock to an unstable person acting out. The thing is, in this matter and many others, when my wife holds such a belief, the only way to argue her actions are wrong would be to challenge her grasp of the situation, her understanding of other people's actions and motivations, and that pretty much amounts to "you are delusional in this matter", which wouldn't go down very well.

This can be described as a Boundary.  Though it may go over better if you state it as OUR home.

Still, you will likely face resistance to a new boundary.  Read up here on extinction bursts which are attempts to impel you to retreat to the prior status quo.  How massive the pushback will be will vary from person to person.  This one may be "the hill she is willing to die on" or maybe not.

I expect and fear she's willing to go to any length to get her way. We'll see. I've been wrong about her reactions many times before.

---

When doing this (inviting my sister and letting my wife know I've done it), what not to do and say?

[Versant]

Is it not YOUR home as well? Do you not have a right to invite YOUR family into YOUR home?

---

I would say that this actually strengthens your case for having your sister over to your house for the event. If it were on neutral ground at a rented venue, then your wife could make a bunch of noise about having a "right" on who to invite and who not to invite. However, in YOUR home, she does not have the right to tell you to keep your family away. Period.

Thanks. Helps a lot to have confirmation that that position is a reasonable one to take.

This behavior is directly attempting to alienate you from your family of origin...without any reason other than irrational dislike of your sister.

That's the thing: my wife is adamant that it's nothing to do with dislike and everything to do with my sister mistreating her. It's just that the "mistreatment" most definitely doesn't deserve that name, and in any case is mostly just healthy person reacting in shock to an unstable person acting out. The thing is, in this matter and many others, when my wife holds such a belief, the only way to argue her actions are wrong would be to challenge her grasp of the situation, her understanding of other people's actions and motivations, and that pretty much amounts to "you are delusional in this matter", which wouldn't go down very well.

This can be described as a Boundary.  Though it may go over better if you state it as OUR home.

Still, you will likely face resistance to a new boundary.  Read up here on extinction bursts which are attempts to impel you to retreat to the prior status quo.  How massive the pushback will be will vary from person to person.  This one may be "the hill she is willing to die on" or maybe not.

I expect and fear she's willing to go to any length to get her way. We'll see. I've been wrong about her reactions many times before.

---

When doing this (inviting my sister and letting my wife know I've done it), what not to do and say?

[Versant]

Made an overture with the "let's give my sister a chance to apologize" plan.
My wife refused to look at my proposal, claiming (correctly, but also a bit unfairly) that I'm thinking about the naming ceremony and inviting my sister there, I'm not aiming to make her life better. She declared my sister's not welcome and there's no need to look at how to let her correct things.

So, things this far are going about as well as one would expect. I remained calm and kind and will see if she reconsiders later, but it doesn't look promising. Probably going to be plan B then (plan A in all honesty) and just inviting who I want anyway.