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.
Some fans would be outraged. Edward and Bella's relationship is iconic, and any deviation from their loyalty to each other would be unacceptable to them.
One possible consequence could be Bella's heartbreak. She would likely feel betrayed and lose trust in Edward. Another consequence might be a rift in the vampire community if the affair becomes known. Tanya could also face backlash from other vampires who support Bella and Edward's relationship.
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.
There are other fans who are interested in exploring different storylines. They see it as a creative way to expand the Twilight universe. They might enjoy seeing how the characters would react in such a difficult situation, even if it's not in line with the original story.
Often, Bella's reaction is one of shock and disbelief at first. She might not understand how Edward could leave her for Tanya. Then, she could either isolate herself from the vampire world completely or she might seek revenge in some extreme fanfiction scenarios. She may also start to question her own self - worth, thinking that she wasn't good enough for Edward.
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.
Some fans might be really shocked and disappointed. Since Edward and Bella are a very iconic couple in the Twilight series, seeing Edward cheat goes against the image many fans have of him. They may feel that it's a betrayal of the original story's concept of their love.
Some fans would be outraged. Carlisle and Esme are a beloved couple in the Twilight series, and the idea of Carlisle cheating goes against the image many fans have of his character. They might see it as a desecration of the original story's relationships.
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.