Haven’t spoken to my ex in months, so why is he now adding my sister on Facebook?

I tried to reach out a few weeks ago (to break NC) sending him a friend request in the hopes we could at least be civil (we go to the same uni, and will be living around the corner from each other in September).

He accepted it but deleted me a few days later (for seemingly no reason). The day after I sent him a message (acting as if I didn’t notice he’d deleted me) asking how he’s been. He read it literally as soon as it sent, but didn’t respond.

This was weeks ago, I haven’t been blocked on anything. I just took it as a sign we weren’t meant to be, not even as friends.

My sister sent him the request MONTHS ago (the day after we split, so she could ask why we broke up. I told him to just delete it) and he suddenly decided to accept it 3 days ago.

They have no mutual friends (he removed me on all social media, though has his other ex on Facebook, but they still talk). He’s never met my sister, and won’t even have his own family on Facebook, cause he’s picky about who he lets see his posts.

He’s also started updating his Facebook a lot more since adding her (changing his profile pic, updating his story, uploading photos, etc) he very rarely, if ever, used to do this, although he was always on Facebook.

The relationship was fairly short (a couple of months), he initiated the break up, but said we could be friends.

Working on the friendship/talking was going well in the days after the break up, until we ran into each other on a night out and ended up sleeping together. The day after he said we needed to give each other space (basically NC, which I’m aware should have been initiated as soon as we broke up).

Why, when he’s seemingly done everything he could to cut himself out of my life, has he decided to add my sister in Facebook, who he doesn’t know but knows I’m so close to?

I don’t think it’s a case of he accidentally accepted, as she’s liked a couple of his posts since, and therefore he’s aware that they’re friends and has chosen not to remove her.
Updates:
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He lives in a different country (still based in the UK) when he’s not at uni (so atm) so I don’t think he’s trying to worm his way in for hook ups with me.
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She saw he liked a post along the lines of “Have you ever seen a photo of someone, remembered all the things you did together, then remembered they’re just a memory and no longer in your life?” The same day he added her. He doesn’t usually like “emotional” posts.
Haven’t spoken to my ex in months, so why is he now adding my sister on Facebook?
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