Juggling a 3 year old, his Auror career and Order duties was hard.
2 years ago one of Sirius' ex girlfriends - name too bitter of a memory to even remember - knocked on his door, shoved a one year old baby girl into his hands and apparated away.
Sirius grew to love his daughter - even when she threw tantrums throughout the night when he couldn't figure out what was wrong. She unknowingly forced Sirius to change, as he slowly realized that he wanted to give his daughter a childhood that he himself never received.
Walburga and Orion's parental love was conditional and cruel, not receiving a punishment or verbal assault of cursed words was seen as a higher level of care. And Sirius could never do the same to his daughter.
But he moved up in his position in the Auror Department in the Ministry of Magic. Sirius was an amazing duelist - a skill he honed all the way back in Hog warts - and working for the Ministry of Magic alongside secret Order missions only made him more skilled, dueling the Death Eaters and fugitives running from the Ministry of Magic as easy as strumming a guitar.
But moving up in the Ministry and the Order missions and meetings becoming more frequent as the Dark expanded it's control over Britain, meant time away from his home - his daughter.
At first Sirius asked Remus or even James and Lily to take care of the girl while he was at work, but because all of his friends also had jobs and were part of the Order, there were times Sirius couldn't get anyone he trusts to look after his precious little girl.
That's how he found himself here - sitting across from you. Honestly, it was Andromeda who recommended you when she heard about Sirius' troubles. Andromeda was always Sirius' favorite cousin, as they both deflected from the Black's Pureblood and bigoted ideals while still at Hog warts, not afraid of being seen as disgraces or disowned.
Sirius looked from the coffee in front of him, to you. You agreed to meet Sirius at his place, when he reached out with the request for babysitting help, promising a pretty galleon for your troubles. You knew that for now, for at least today Sirius was testing you to see how you would do; but most importantly, how his daughter would react to you, if she would adjust to you.