Ginny might be extremely embarrassed. She would probably blush deeply and try to hide behind Harry at first.
Another reaction could be that Ginny is worried. She knows how important Sirius is to Harry, and she might be afraid that Sirius would disapprove. So she might start stuttering out explanations about how they really like each other and how they were just caught up in the moment. She would hope that Sirius would understand.
Sirius might react with a mix of surprise and protectiveness. He would be like an older brother or father figure. He'd make sure they were safe and not doing anything too risky.
They might start by saying that their relationship is based on deep love. Harry could mention that Ginny has been there for him through a lot of tough times and they just got carried away in a moment of passion. Ginny could add that they respect each other and didn't mean to disrespect Sirius or anyone else.
In many fanfictions, if Harry and Ginny were caught by Sirius, Sirius might be initially protective and concerned. He could question their actions, especially if they were in a situation that seemed dangerous or inappropriate for their age. For example, if they were out after curfew or in a restricted area. Sirius might give them a stern lecture about being responsible and the importance of following rules, while also understanding their adventurous spirits as he was quite the rebel himself when he was young.
In fanfiction, if Harry and Ginny were caught in bed by Sirius, there could be a lot of shock and embarrassment. Sirius might initially be angry or disappointed as he could see Harry as a younger person under his protection. He might give them a stern lecture about responsibility and propriety. Harry and Ginny would likely be extremely mortified. They would have to face Sirius' judgment and deal with the awkward situation. This could also potentially cause some rifts in their relationships with Sirius and among their friends if word got out.
Harry might blush and stammer. He could say something like, 'Sirius, it's not like that.' and then quickly try to change the subject.
One possibility is that Sirius could be initially shocked. But then, being the understanding person he is, he might give them his blessing. After all, he has seen how Harry has grown and Ginny is a great girl.
In non - lemon fanfiction, their relationship can often be one of mentorship. Sirius, with his experience and knowledge of the wizarding world, could offer guidance to Ginny. For example, he might teach her some advanced spells or give her advice on dealing with the darker aspects of magic.
Maybe Harry noticed Ginny acting strangely in the fanfiction and he wants Sirius' advice. Sirius has a lot of life experience and wisdom, so Harry might think he can help figure out what's going on with Ginny.
In some fanfictions, Ginny might use her love and emotional support to help Harry heal. She could be there for him, listen to his problems, and give him a sense of normalcy. Sirius, on the other hand, might share his own experiences of dealing with dark magic and trauma. He could teach Harry how to better protect his mind from the after - effects of his battles, like using Occlumency in a more effective way.
They might try to deny it at first, or come up with some sort of excuse. Maybe they would claim it was a misunderstanding.