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At bare minimum my boundary would be my wife does not attend parties at her sister's house unless it's a family gathering such as Christmas and I am present. Even though I do not like my sister-in-law and her husband there is no way I would be comfortable with my wife going to a party at their house without me present

Think about this. Your wife doesn't know if her marriage is going to survive so cutting ties with her sister and brother-in-law would be foolish because if her marriage does end she will want and need her sisters support so there is probably little chance she would entertain the idea of cutting ties

Your wife does not have to accept or agree to your boundaries same as you do not have to stay in a relationship with her. You are not telling her what she can and cannot do, you are telling her what you will and will not accept in a relationship. At that point she gets to decide which is more important to her

I can easily understand why them cutting ties with the OM would make your life easier because you would not have to worry about your wife being at her sister's house and the OM shows up but what you want means nothing to them it appears

Do not take revenge, my dear friends, but leave room for God’s wrath, for it is written: "It is mine to avenge; I will repay,"

That’s the verse your user-name refers to.

So I wonder how you could approach this situation in a way that could be supportive to your wife, her extended family and your marriage.

I generally believe in total honesty. I can imagine you and your wife agreeing on some statements like:

"We are working at reconciling our marriage. We might not be 100% certain how we will do that or if it’s possible, but for now we are trying. One thing we can agree on is that our mutual infidelities are part of our problem and something we need to deal with. Part of dealing with that is that neither of us interact in any way or form with any of our former affair partners. If interaction happens, we minimize damage by getting out of that situation and sharing with the other. This is mutual, and is agreed on by us both as something required in order for us to possibly reconcile."

Frankly – if she can’t agree with the above then you two aren’t in reconciliation.

Second statement:

"We can agree that it can be important to have a good relationship with close family, like your sister and BIL. I realize they received you once during a difficult time in our marriage, and I should appreciate they took care of you. I do however have an issue with their friendship with OM, and we need to find ways to mitigate that concern."

Third statement is one you agree on and present to SIL and BIL:

"We appreciate your friendship and your assistance to our marriage in these difficult times. We are working at reconciling and think it would benefit us to be able to have you in our lives. However we do NOT want to interact in any way or form with OM. We ask that you ensure that when we are with you (either singularly or as a couple) that you do not talk about OM, events OM is at or accept him to your presence when we are there. It is imperative for our marriage that neither one of us interacts with OM. What you do outside of that is not our concern. We appreciate your help and compliance on this issue".

If you two can agree on all three statements (or more appropriate versions for you) then you two as a couple have a good chance to reconcile. Her family can react in a couple of ways:
They can tell you and your wife that you two do not control their social life and refuse to take your wishes into consideration. To me I would view that in a comparable way as I would friends who might refuse to not serve peanuts due to my peanut allergy (just a simile – love peanuts!). I would see this as a clear sign that they aren’t positive for the marriage.
They can accept the statement, but largely/partially ignore it. Might ask you over for dinner and talk about OM and work, might have a party and he’s there serving the drinks or whatever. That too should be a sign, and since the sign is the emotional equivalent of a full-size billboard your wife will recognize that they are NOT helping.
They can be supportive and agree to your statement and respect it.