I just know Jin is the Always Worried About Her, omg, tell me ALL about this girl you like brother to Himiko, and Dabi is the nonchalant, we don't hang out & you piss me off but I'll kill someone for you brother, and Tomura is fully the What The Hell, Sure @ anything Himiko says/wants + lots of quality time brother.
I picture Tomura being so compliant with everything Himiko wants to do... Like, ok kid, let's read romance manga out loud together while you smear lipstick on my face idec.
She can probably play fight with Jin and Dabi safely and bicker with them for fun. I know Dabi has undone her hair buns more than once. If they get bitten in the process, that's their fault for messing with the wrong one! Tomura doesn't have that kind of brother built. He probably gets kind of nervous and jittery and doesn't enjoy it/doesn't KNOW how to enjoy it. He's just not very good at play fighting and it makes him tense.
Himiko is really affectionate and sweet and I imagine Dabi rejecting that, while Jin basks in it wholeheartedly. He's pretty affectionate himself for a guy that was lonely for some time; like, we can see how touchy he is even with Dabi. She feels safest/happiest being shown affection from Jin. I think Tomura was probably freaked tf out the first time she hugged him because he hasn't been hugged since he was FIVE. He kind of awkwardly lets it happen.
I'm just having so many Himiko thoughts.... My cutieful daughter.
When it comes to Mag (bc she obviously loved her), I think they were able to talk on a deeper level about stuff that mattered. Her, Jin and Mag, probably, because Horikoshi once described her relationship with Jin where he rambles and she listens to him and nods and smiles politely. I think to reign him in and also shed some light on Himiko's worries and trauma, Magne was a very crucial part of their relationship. She probably balanced things out a little bit and made Himiko open up some more.