To tell SIL...

[Soitis83]

So I've been told that BIL is flirting at his work and saying gross things along the lines of "I would F the S out of her.." when a woman walks in and out of the office.
Do I tell SIL? Is this just office banter?
I've also been told he flirts with the women at every work party.
She honestly thinks the sun shines out of his butt. Me however, am not surprised at all, he's always been slimy imo. I really, really wouldn't be surprised if an affair comes out at some point...

It's not office banter it's degrading and it's definitely NOT flirting.
how do you know he uses those actual words and why were you told?
someone needs to report him to HR.

I hate blokes that think that is example- its like going back to the 70s but I wouldnt be saying anything to SIL unless you had suspicions of an affair. I have no doubt if he talks like that in the work place he likely says similar things about women on the TV -

agree with @MollysBrolly if it happened in my work place he would be reported

oops apologies for my typos above!

100% this

That would be gross misconduct in my workplace

How do you know this, who told you?

Not your relationship, workplace or business

I agree with this. I think that all you can do is to support her when he either loses his job, gets caught cheating or leaves her.

He sounds like a delight.

Who has told you? Are they reliable?

Whoever is actually experiencing this at work needs to report it via the correct channels.

but I don’t think anything is to be gained by you getting involved in the gossip.

Yes the person who told me is very reliable. Absolutely hates conflict and the reason they told me is because it's really been getting to them. They don't mind if I say something as they just want it to stop but I'm unsure. As people have stated it's not my business. It's just laying heavy on my conscience at the moment.
This man is a manager there..

The women who he says stuff about don't know as it's when they've left the room apparently. But there are younger girls who flirt at parties

So it's your BIL and SIL I'm assuming that one of them is your partner's sibling.

Have you told your partner?
Have his co-workers reported him to HR/senior management?

I'd say nothing as you only have the information second hand.

Is BIL your husband's brother? I'd tell DH to have a word with his brother as rumours were swirling but I wouldn't tell SIL that.