Sirius didn't go to the Potter house after running away from his parents, not right away.
He went to diagon alley, James found him there, sleeping in the alley by his favorite quidditch store.
Euphemia and Fleamont were talking to the Weasley's, outside of Gringotts when James showed up with a poor and sick looking Sirius who looked like he hadn't been fed in days, which may have been entirely possible.
Sirius didn't believe it. the Potters would let him live with them, for however long he needed or wanted? Without payment? Everything costs money though??
He waited anxiously for the switch as soon as they were out of earshot to complain and criticize "three kids already AND another on the way" instead it was "Molly truly glows, she's made to be a mother!" Maybe he'd know what they really thought of the Weasley's once they were home?
He waited with anxiety to be told how unkempt and horrible he looked.
Effie didn't scold him. he got a hearty meal before a bath and a nap in Jame's room. Just for now "She'd make up a bed for him for tonight."
Sirius waited or them to get mad that when he was more comfortable sharing the room with James, after she'd gone through all that work to prepare a room for him, ready before he even woke from the sorely needed map.
Waited for the switch of personalities as soon as they're no longer in public
Waited for the anger that would surface as soon as they were home
For them to blame him
How could they be happy all the time?? Surely it was an act. Surely they'd stop acting like they're ok with him staying after a day. After two days? After a week? After he messed up? After he broke something? After he was too loud? After a month? How long were they gonna keep the act going?
Even when Fleamont was overworked and late for supper he wouldn't come in yelling and slamming doors, insisting everyone was far too loud. He'd tiredly smile as Euphemia warmed the food she set aside for him and she'd come over with the plate and a soft kiss.
Even when Sirius caused a mess carrying dishes to the sink and the bread was burnt and the dishes piled up and the laundry getting rained on out on the lines Euphemia smiled and they had dinosaur shaped burnt-scraped-off bread for dinner with jam from a jar that had a broken edge.
Even when James grumbled about being made to do chores and a splinter in his finger swelled, he smiled and joked around.
The Potters were happy?? They didn't fake everything for the public eye??
Euphemia didn't tell James to "remember who he represents" she reminded James to be careful and to take a coat, just in case
Euphemia reminded Sirius that he should take a coat too, James had an extra one Sirius could have.
Dishes and meal cleanup weren't a punishment. everyone chipped in. Fleamont started the dishes while James cleared the table and Euphemia put the extra food in containers "we should visit the Longbottoms with a casserole." "Indeed, I quite enjoyed the one with the cheese you made last week, I'm certain they'd enjoy that one" "but only if you put some of that special sauce on top too, Ma." Easy, casual conversation??
They all went to sit together in the living room after dinner and they didn't all just go separate ways on their own. Checkers? Newspaper? Sewing? They did it alone but together but in the same room.
Fleamont asked for his opinion.
Euphemia asked for his help, even after he made a mess, last time.
James wanted to spend every earthly moment with him, it didn't matter what they were doing, just as long as we're together.
All three told him they were happy they got to spend time with him, even if the reasons for him being there were sad.
They let James keep the kitten he found, at least till they found the owner.
Euphemia praised Sirius' singing, whether French classical or David Bowie.
He got to help decorate for Christmas this year. The hundreds of random and uncoordinated ornaments that each had a story. The year Fleamont and Euphemia started dating, and how this chip was because she threw it at him during one fight, but they laugh about that story now. James' first Christmas, he had the poofiest hair as a baby.
James got to fly his broom inside to hang the garlands and tinsel from room to room. James trying to hide the mistletoe, and smiling when he failed.
Sirius hadn't wanted to get married, before. He didn't want a marriage like his parents.
Sirius wanted to love like the Potter's, to laugh every morning with his best friend. To keep mistletoe in his pocket for nearly a whole month, just to see his partner's smile every time.
Sirius hopes he and someone special can love each other the way Euphemia and Fleamont love each other.
He wants to be like the Potters.
He'd make himself love like the Potters not like the Blacks.