It would probably become more strained. Sirius may start to distance himself from Dumbledore. He might not follow Dumbledore's orders as readily as before.
It could be due to Dumbledore's decisions regarding Harry. Sirius is very protective of Harry. If he thinks Dumbledore is using Harry in some way for the greater good without fully considering Harry's well - being, he would be fed up. For instance, Dumbledore's decisions about Harry's participation in the Triwizard Tournament could be a point of contention for Sirius.
Well, at first, Sirius would be in a state of shock and panic. But then he'd have to step up. Their relationship would shift from one of equals (in a sense, as Harry was growing up) to a caretaker - cared - for dynamic. Sirius would have to learn about Harry's likes and dislikes as a baby. He'd sing lullabies to him, comfort him when he cries. This new experience would make their bond stronger in a very different way. Harry might not remember all of it when he's turned back, but there would be a subconscious change in how he feels towards Sirius, a deeper sense of security and love that stems from those baby - days when Sirius was his whole world.
Their relationship would be severely damaged. Harry would likely feel betrayed by Sirius. After all, Sirius is supposed to be one of his protectors. It would be a huge breach of trust. Sirius, on the other hand, would probably be filled with guilt if he realizes his mistake. He would have to work hard to regain Harry's trust, if that's even possible in the context of the fanfic.
The relationship could become strained. Sirius might start to view Harry differently, with a bit of wariness. He might not trust Harry as much as before in terms of making good decisions. But it could also lead to a more open relationship where they talk about things they previously avoided, like relationships and feelings.
It could develop in a complex way. The spanking incident might initially create tension between them. Harry could be angry or resentful towards Sirius at first.
The relationship could also take a more complex turn. Other characters might get involved in their conflict. Some might side with Sirius, thinking Harry was in the wrong, while others might try to mediate. This outside influence could either help or hinder the mending of their relationship, depending on how it's handled in the fanfic.
In this type of fanfiction, the relationship is often explored through emotional depth. For example, Sirius might be portrayed as a mentor to Harry, guiding him through the difficult world of magic. It could also show their shared experiences of loss, which bonds them closer. Maybe Sirius shares his memories of James with Harry, strengthening their connection.
His relationship with Sirius would be extremely strained. Since Walburga was Sirius' mother and they had a very difficult relationship, Sirius would likely see Harry as a product of his mother's influence. Harry, on the other hand, might be initially hostile towards Sirius due to being raised with Walburga's views.
Well, their relationship would be in tatters. Harry would probably avoid Sirius like the plague after such an event. Sirius, on the other hand, would be at a loss for how to deal with what has happened. He might feel betrayed, even though he knows Harry was forced. This would create a huge rift between them that would take a lot of time and effort, if ever, to mend. It would also change the way they interact with other characters in the fanfiction, as their relationship was an important part of the overall story dynamic.
Sirius would probably be overjoyed at first. Harry is his godson and he cares deeply for him.