To keep this to myself if it burdens someone else by telling?

[TheRawPrawn]

My mother in law died 8 years ago after a five year battle with cancer. My husband really loved his Mum, and he was heartbroken. My PIL were married for 35 years, FIL can be a bit of jerk and they didn’t have the nicest relationship in my opinion.
About 6 months later, FIL started dating her close friend who had worked for them in their business. At the time, it did feel quite soon, but he was really happy and we liked this lady as a person. They broke up suddenly and bitterly a couple of years ago, FIL was very evasive about the reasons.
The relationship between FIL and my husband has deteriorated. FIL has made several upsetting decisions, had callings out with family members and is just generally a difficult person to get along with. But he is still my husband’s Dad, and our children’s only grandparent. He lives quite far away, we haven’t seen him since the pandemic started, we can’t have him stay (he’s an anti-vaxxer and being isolated together in a house if someone got covid would be a nightmare). But we’ve offered to put him up in a hotel on our road and do activities out of doors. He hasn’t taken us up on it, his proposed anti vaxxer position is he won’t get vaxxed and he won’t travel.
I’m just setting the scene that relationships aren’t at their best.
I was talking to my sister in law on the phone recently, and she told me (more assumed I knew) that FIL’s relationship had started when my MIL was alive and in palliative care. My husband’s grandmother saw them together. I suspected that it had started earlier than the official story, but not while she was actually alive.
I know if my husband knew, he’d probably cut him off. I tell my husband everything and I’ve known this for a few months and haven’t told him. I feel like I’m lying to him.
But what help would it be for him to know? It would just cause him pain and a falling out.

What should I do? Should I tell him?

U - tell him
NU - keep to yourself

Would anyone benefit from you telling your DH this now?

Clearly not.

It would hurt him, if he doesn't already know. Best to stay out.

@Rodion

I think the most painful thing would be if he ever found out that you knew and didn't tell him, so on that basis I'd tell him. But it's horrible have info like this where it's lose lose so you have my sympathies.

Agree. If you were the only person who knew, I would say don't mention it. But your DH is bound to find out.

I'd say something along the lines of @Cherryana's post too, but would just clarify that your SIL doesn't know this, she thinks this.
For me it's less about your DHs relationship with his dad and more about his relationship with his sister as at the moment it's been affected by something that your husband has no clue about. Whatever the truth of the relationship, I'd want the chance to clarify to my sister that it wasn't me being cool with a potential affair, but that I had no clue there was a potential affair.