Watching an abusive relationship unfold in front of me may be more than I can take, even if Sir is good at hinging.
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My issue with Sir is that he has let her ruin every relationship he's been in outside of theirs.
The two sentences seem incompatible. Or perhaps you are saying Sir is usually good at hinging, but this toxic mess is overflowing.
Others have pointed out that you don't have to have a relationship with Grace. And you have said that you have stopped it. Now your issue seems to be that you can't watch an abusive relationship unfold and do nothing. Part of the dilemma is that the relationship is between them.
Equally clear is that even if the relationship is between them, it is adversely impacting your well being.
One possibility is the very inferior either/or. Either end that relationship or we are over - which is where your mind is at. Of course, you are thinking it the politically correct way that you end it because you can't handle them. But not like if he offered to end that, you'd end it anyway.... so let us be clear what your mental process is, even if your handling of it is elegant.
An alternative that occurs to me is that you are troubled because you care about Sir and you don't like feeling helpless to do anything while he is being abused (in your perception). You aren't exactly going to be happy to lose him.
Have you considered that by merely
existing in Sir's life, you're reducing the percentage of abuse in his relationships? Even doing nothing, you are actually preventing Grace from making abuse a greater share of his relationships. It is actually more than you'd achieve by going missing from the scene - if Sir being abused is the issue.
There are many ways to intervene in abuse. An important one is to provide a clear contrast, so the person being abused doesn't lose him/herself to thinking that is "normal". You are providing that by simply existing in Sir's life and now that you know this is a method of intervention, you can do it with explicit intention.
It provides Sir a space to use to exit abuse if he needs to. If nothing, it provides a useful benchmark for how a non-toxic relationship can be, so that he can choose what he wants for himself.
Of course, if your relationship with Sir isn't all that major a part of your life, it may be simpler to simply end it and move on. Given that your problem is not with Sir, but with Grace, for something like this, I'd decide based on what would hurt me least. Losing Sir or knowing Sir is with Grace?