Decide which relationships you actually want to keep
Not every in-law relationship deserves a preservation effort, and treating them all the same is the first mistake people make. Sit down, literally with a piece of paper if it helps, and write the names of the people from that family who mattered to you independently. Not because they were useful to the marriage. Not because losing them looks bad. Because you genuinely liked them, or loved them, or built something real with them over years of holiday dinners and ordinary Tuesday texts.
Then ask yourself one honest question about each name: does this person make me feel seen, or do they make me feel like evidence in a case? Some in-laws will never be able to separate you from the divorce. They will always be, in some low-level way, loyal to the version of events that makes their family member the protagonist. That is not a character flaw you can reason them out of. It is just true.
The relationships worth keeping are the ones where both people want the same thing. You cannot maintain a relationship alone. If his sister is not returning your calls, that is information, not a problem to solve harder. Let the list get shorter. The ones who are still there at the end of that honest accounting are the ones worth the next steps.
Set the terms of contact before feelings set them for you
Here is what tends to happen when people skip this step: they stay in a loose, undefined connection with an ex's family member, texting sporadically, liking photos, showing up at the odd birthday, and then one day something shifts. Someone mentions the ex's new relationship at brunch. Or asks a question that is really not a question. And suddenly the whole thing collapses under weight it was never built to hold.
Before that happens, have a simple, quiet conversation with the person you want to stay close to. You do not need to make it formal. You are not drafting a treaty. You are just saying out loud what both of you are probably already thinking: I value this. I also need this to be its own thing, separate from whatever is happening with him. Can we do that?
Most people, when asked directly, will tell you exactly where they stand. His mother might say yes, absolutely, and mean it. She might say yes and not mean it. You will be able to tell. What you are looking for is a relationship that does not require you to manage information, perform wellness, or avoid entire topics to keep the peace. If the price of staying connected is constant self-editing, it is not actually connection. It is just proximity with extra steps.
Decide on a rhythm that feels sustainable. A monthly phone call. A standing lunch. Whatever fits the relationship. Vague intentions fade. Specific plans do not.
Create a boundary around what you share about the divorce
This is the part that trips people up the most, because it feels dishonest, and you are not a dishonest person. You do not want to lie. But sharing the full, unfiltered reality of your divorce with your ex's family is almost never a good idea, no matter how close you were, no matter how clearly they have chosen your side.
Think of it less as hiding and more as protecting the space. What you tell her aunt about what happened between you and her nephew becomes part of that aunt's relationship with her nephew. You cannot control what she does with it. You cannot un-say it. And even the most well-meaning family member is not a neutral party. They cannot be. That is not a criticism. It is just the geometry of the situation.
A useful phrase, one you can practice until it becomes automatic: I am doing okay, and I am trying to keep the details pretty private. Most people, when told this clearly and warmly, will respect it. The ones who push past it are telling you something about whether this relationship is actually safe.
What you can share: how you are feeling in general terms. What you are looking forward to. What you have been doing. The ordinary texture of your life. That is enough. That is, actually, what real friendship is mostly made of anyway.
Mark the shift deliberately, so it does not just evaporate
One thing research on grief consistently shows is that the regular passage of time is not enough to process a real loss. Something deliberate has to happen. A marking. A moment where you acknowledge what changed and what is still here.
This applies to family relationships too. If you had a tradition with his parents, Sunday dinners, a standing movie night with his sister, those rituals defined the relationship, and they are gone now. You need new ones, or you need to consciously close the door on the old ones instead of just letting them fade into awkward silence.
If you are keeping a relationship with someone from his family, build a new ritual that belongs to just the two of you. Not a recreation of what you used to do together as a foursome. Something that starts now, in the shape of what you actually are to each other, which is two people who chose each other when no one was requiring it. That choice deserves its own form.
If you are letting a relationship go, let it go on purpose. You do not have to send a letter or make a speech. But privately, for yourself, you can acknowledge that this person mattered, that losing them is a real loss, and that you are marking it rather than just letting it quietly disappear. Research suggests that this kind of deliberate acknowledgment, even a small private ceremony, gives you back a feeling of control that loss tends to take away. It works even if you feel a little silly doing it.
Protect the relationship from becoming a back channel
This is the one that sneaks up on people who have the best intentions. You are staying close to his sister because you love her, genuinely, and then one evening she asks something small, just a throwaway question about whether you know if he is seeing someone. And you realize, with a quiet sinking feeling, that you have become the person who knows things about him, and that she is starting to think of you that way too.
Or it goes the other direction. You find yourself asking her questions you have no business asking, using the warmth she has for you to find out things you could not find out any other way. Neither version is fair to her. Both will eventually corrode the thing you are trying to keep.
The protection is simple but requires repetition: every time the conversation drifts toward your ex, redirect it. Not dramatically. Not with a speech about boundaries. Just, oh I actually have no idea what he is up to, but tell me what is happening with you. Practice it until it is the most natural thing you say. The relationship you are maintaining is with her, not with a version of him filtered through her eyes. Keep those two things separate and you have a real chance. Let them blur and you will lose both.