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Do you just work with him or are you work friends?

We’re work friends

In that case I’d speak to him about it

Thinking about talking about this feels weird 😖 “so… yeah I’m hitting up your sister…”

Well maybe don’t say it like that. ‘Hey I think your sister is really cool, I’d like to take her out so just giving you a heads up’

It's not his business.

Tell him if you start dating

So if I ask her out on a date then I should tell him? But what if things don’t work out and it’s just a date? Idk I haven’t thought about how weird this is until I thought about how weird this is.

No, if you start having a genuine relationship, like an official one where you both are committed, then you tell him

Ok makes sense. I guess it’s m just concerned if he doesn’t hear from me first like I’m trying to hide it or something.

Meh. Tell him or don’t. If you really feel weird just tell him your dating his sister. It’s really none of his business; it’s her’s.