Rest of it

Hey, i dunno if im dumb or what but i dont quite understand the 4th story. I just felt so sad, can somebody enlighten the poor me.

I didn't understand the second one. Who did he love at last?

It's insinuated he was obsessed with yumeji. While they were fucking, ml takes a picture of yumeji, then adds it to an album of photos of yumeji.
We can assume yumeji doesn't know about the album, because he's not smiling or looking at the camera in any of the photos as if he's unaware they're being taken. This part makes me think Yuu has been into yumeji the whole time.
The way Yuu (the ml) literally smiles when Yumeko catches them fucking, then the way he twisted yumeji's arm to keep him from running after his twin, and especially the end when Yuu says "how will I make [yumeji] cry next.." just backs it up. Yuu WANTED Yumeko see him fucking her brother (which he wouldn't do if he liked her). The way he twisted Yumeji's arm especially bothered me too. I interpreted it in two ways – 1) Yuu's possessive over Yumeji. He both wants and will monopolize him 2) on top of that, he LIKES hurting Yuumeji. He straight up told yumeji he wants to hurt him and from his actions and words, it's clear that causing yumeji's emotional and physical pain doesn't discomfort Yuu. He's only concerned with how Yumeji's pain can entertain himself. Yuu doesn't give af about Yumeko. He just used her as a tool to hurt Yumeji.

The reason why we get confused and think he does, is because of the unreliable narrator. Most of the story is seen from Yumeji's perspective and yumeji believes Yuu loves Yumeko.
Objectively, there's basically no proof that he does, besides Yumeji believing so, and Yuu saying he does (AFTER Yumeji says so first). Don't believe yumeji cause he's insecure and confused. Also don't believe Yuu because he's a crazy sadist who will lie to watch Yumeji crumble.

– Boom, the evidence is gone and in conclusion, Yuu doesn't love either of them. he's just obsessed with Yumeji and likes causing him pain

I forgot to put a spoiler alert I am so sorry oops (´°ω°｀)