Bella might be extremely hurt and confused. She would likely try to convince Edward that she didn't cheat.
In some fanfiction, Bella might go into a state of depression. She loves Edward deeply, and his lack of trust could shatter her. She could stop taking care of herself, lose interest in her normal activities, and just mope around. She may also start to question their relationship and whether it was all a mistake if he could so easily believe she cheated without proper proof.
Well, Edward could start to distrust Bella completely. He may constantly question her every move and word, leading to a lot of arguments between them. For example, he might not believe her when she tries to explain things. This could also cause a rift in their relationship with other characters, like their vampire and werewolf friends who might get involved in the drama. The whole dynamic in their fictional world would change, with a lot of tension and unease.
Together, they would likely try to make Renesmee understand that their relationship is special and that her presence is also special. They might give her a hug or some form of affectionate gesture to show that they are a family unit and that her walking in is just a normal part of family life, even if it was an intrusion on their private time.
Edward might be initially shocked. He has always seen Bella as his own and this sudden change could be hard for him to accept. He could become withdrawn and moody, similar to how he was before Bella entered his life.
Edward might be initially shocked and heartbroken. He has been in love with Bella for a long time, so her leaving for Jasper would be a huge blow. He could become withdrawn and moody, brooding over what went wrong.
He could be devastated. Edward is deeply in love with Bella, so he might go into a state of depression. He may isolate himself from his vampire family and friends, constantly replaying in his mind what went wrong in their relationship.
Edward would probably be very confused at first. He would try to understand why Bella would leave him for Rosalie. He might start to analyze all their past interactions, looking for signs that he missed. Eventually, he could become angry, not at Bella directly, but at the situation. He might also feel a sense of betrayal, not just from Bella but also from Rosalie in a way, since Rosalie is 'taking' Bella away from him.
Bella would be heartbroken. She has put so much trust in Edward, and this act of cheating would make her question everything. She might become angry and lash out at both Edward and Tanya. She could also become very sad and isolate herself from others.
She could be heartbroken. After all, she's pregnant and the person she loves has left her. She might feel abandoned and alone, crying a lot and wondering why Edward would do such a thing.
Often, Bella reacts with intense heartbreak. She might isolate herself from her friends and family, feeling like she can't trust anyone anymore after Edward's betrayal.
Bella might initially be terrified. She is a human being kidnapped by a vampire, after all. She could struggle and try to escape, using her wits and any available resources. However, since she has feelings for Edward in the original story, part of her might also be conflicted, wondering if there is a good reason behind his actions.