I'm just centring this one around the Hol and fluffy things since my recent poll about what y'all want said that most want Fluffy domestic stuff and in second y'all wanted headcannons.
Due to their heightened senses, demons can hear a beings heartbeat from far away. The brothers all keep track of each other's heartbeat and Lucifer made sure they could still sorta hear Belphegor's from the attic but it would sound far away.
The brothers don't usually take turns actually cooking. It's usually Lucifer or Mc cooking but if they don't it's just take-out. Mammon has been banned from the kitchen twice because things keep catching fire. His attempts were appreciated.
Diavolo sometimes stays over at the House of Lamentation so he can feel less lonely. The brothers just accepted this, the only one who still complains is Belphegor.
In the celestial realm, the seraphim would make jokes about Lucifer secretly being a fae because he "stole" Mammon from Micheal.
Mammon and Beel help Asmo redecorate his room which happens at least twice a month. Satan tries but Asmo usually just throws him into a fashion show to make him wear his jacket correctly.
Satan pushed Lucifer off a roof twice.
There's some unusual and unspoken rules in the House of Lamentation that don't make sense until they are experienced. For example, rule 6: if Lucifer is passed out on the floor do not wake him, just leave a blanket and move on. This is because Lucifer may get startled and throw whoever wakes him.
Mammon sometimes brings Lucifer snacks if he's been in his office too long.
Asmo has dragged Leviathan out to multiple "Asmo Nights" and Asmo helped Leviathan get a date once. It didn't work out romantically but they still play Minecraft together.
Satan had a goth phase, Lucifer and Mammon are the only ones with pictures.
Leviathan makes money off of art commissions and proofreading some literature. This is how he seems to always have money to pay Mammon.
Satan sometimes helps Lucifer by giving him blackmail he collected on the House of Lords. Satan does this willingly because he hates some of those bastards more than Lucifer after some called Mammon and Asmo slurs. They're lucky they are nobility otherwise they'd be very dead.
Asmo has Pinterest boards for all his siblings as well as themed collages centred on his siblings that he rotates through daily.
Mammon's Home Screen is a photo of Mc with ice cream on their face laughing. His lockscreen however is a shot he got of Lucifer being angry.
Belphegor has a designated spot on the couch that he always sleeps in and the brothers always move him there.
Satan can be picked up and moved around by his siblings like he's a cat and he has been. The brothers did it so often when he first formed that he thought this was normal.
Lucifer still picks up his siblings like they're children. He realised one day that he just stopped picking them up and so he started doing it again because he could.
Mammon bought Satan a baby harness but so Satan could carry around a cat in it. Satan also a backpack with that little window and air-holes that are for cats.
