Sirius might immediately rush to Harry and give him a big hug. He's like a father figure to Harry, so physical comfort would be his first instinct.
He could start asking Harry what's wrong in a very concerned tone. Sirius is a protective character, and he would want to know the cause of Harry's distress so he can fix it. He might sit down with Harry and listen attentively to his problems.
Sirius would probably be shocked at first. Then, he'd likely feel a deep sense of sadness and guilt. He might think that he should have been there more to protect Harry from getting those scars.
He might get angry. The scar is a symbol of his failure, so seeing it could trigger his rage.
He might be shocked. Snape could be taken aback by the depth of Harry's emotions and experiences in his memories.
He might be initially shocked. Snape has a complex relationship with Harry, and seeing those scars which are a mark of Harry's past and his connection to Voldemort could stir up a lot of mixed emotions in Snape. Maybe he would feel a pang of guilt, as he has some role in Harry's life due to his connection with Lily.
Maybe Harry is crying because he misses his parents. He often feels the pain of their absence deeply, and in this fanfic, when Sirius sees him, that could be the cause.
He could be stunned. Draco has probably heard about Harry's scars but seeing them in person is a whole different thing. It might make him speechless for a moment.
Often, the fanfic will show Harry crying while remembering all the good times with Sirius. It might be a flashback of them joking around or Sirius giving Harry some important advice. These memories make his crying even more heart - wrenching as he realizes he'll never have those moments again.
He's probably furious. Snape has high standards for student behavior, and smoking is definitely not acceptable in Hogwarts. His face might turn red with anger, and his eyes would narrow as he glares at Harry.
Sirius would probably be very concerned. He might immediately ask her what's wrong and offer her a handkerchief to dry her tears. He has a bit of a wild side, but he also has a big heart, especially when it comes to those he cares about or sees as in need of help.
Sirius would likely be extremely angry. He already has a protective streak towards Harry. He might immediately confront those he suspects of the abuse, whether it's the Dursleys or others at Hogwarts. He would probably also try to remove Harry from the situation as quickly as possible.