I'm not sure specifically as there could be many different plots in such a fanfiction. It might involve Sakura having various interactions with the Uchiha clan members, perhaps she forms new friendships, or gets involved in some adventures related to the Uchiha's unique abilities and their history.
The main character is Sakura. For the Uchiha side, Sasuke is a no - brainer as a main character in this fanfiction. But it could also include some other Uchiha characters that are not as prominent in the original series. For instance, a young Uchiha who is just starting to discover their Sharingan powers and Sakura becomes a mentor - like figure to them. Or perhaps an older Uchiha who has survived some great tragedy and Sakura helps them come to terms with their past, all while exploring the unique culture and abilities of the Uchiha clan.
In a 'sakura brother killed the uchihas' fanfiction, the plot could start with the Sakura brother having a hidden power or ability that the Uchihas unknowingly pose a threat to. He might see the Uchihas as a danger to his sister or to the village in general. And as a result, he decides to take matters into his own hands and eliminate them. Or it could be that he is misinformed about the Uchihas and thinks they are evil, leading to his actions.
I haven't come across a 'sakura brother killed the uchihas' fanfiction. It could be a really unique and perhaps not very mainstream idea. Fanfiction often has a wide range of themes, but this seems rather unusual. Maybe it's a very specific creation within a small group of fans.
Sakura would probably be angry at herself for being captured. She's a proud ninja. So she would start looking for weaknesses in Itachi's hold on her. Maybe she would try to engage him in conversation to find out his motives, all the while formulating a plan to break free. She could also try to send out some sort of signal to her comrades if possible.
She would be both curious and empathetic. Curious about how the baby ended up where she found it and empathetic towards the baby's situation. So she would start to form an emotional bond with the baby right away. She might talk to the baby softly, trying to soothe it, and then start making plans on how to best care for it in the long run.
Another reaction could be that Sakura tries to follow Sasuke. She believes that she can change his mind or that she can be of help to him. She might leave her own responsibilities behind for a while to chase after him, thinking that their bond is strong enough to overcome whatever is making Sasuke leave.
In some fanfictions, Sakura may be determined to find Sasuke and bring him back. She could set out on her own journey, facing various challenges along the way, and growing both as a person and as a ninja in the process of trying to reunite with him.
She might be heartbroken. Sakura could initially be in shock and then fall into a deep sadness. She may blame herself for Naruto's leaving, thinking that she could have done something differently.
Sakura would be over the moon. She would blush deeply and look at Sasuke with so much love in her eyes. She might be at a loss for words for a moment, but then she would find her voice and tell Sasuke that she has been waiting for this moment her whole life. She would also feel a sense of pride that Sasuke has chosen her, after all they have been through. And she would immediately start thinking about how to share the good news with their friends, especially Naruto, who has been like a brother to her and has also been a part of their journey.