Well, Anna would be frantic. She'd immediately set out to find Elsa, not caring about the danger. She might even team up with the Guardians without hesitation. Kristoff would be right by Anna's side, using his survival skills to try and track down Pitch's location. Olaf, being the innocent and loving character he is, would be heartbroken. He might try to cheer Elsa up from afar by singing or reminiscing about their good times, hoping it reached her somehow. The trolls, if they found out, would offer their wisdom and perhaps some magical aid to help in the rescue of Elsa from Pitch's clutches.
The people of Arendelle would be in a state of chaos and mourning. Elsa has been their queen and their protector. Some might be afraid that without Elsa, the kingdom would be vulnerable to new threats. The trolls, who have known Elsa since she was a child, would also be deeply saddened. They might try to find a way to bring her back, using their own magic and knowledge. Sven, Kristoff's reindeer, would sense the change in Kristoff's mood and would be more subdued, staying close to his owner during this difficult time.
Elsa's reaction would be complex. She has spent most of her life in isolation, fearing her own powers. A kiss from Hans would be something she never expected. She might feel a sense of betrayal towards her sister Anna at first, since Hans had once tried to harm Anna. But as she realizes her own feelings, she would have to face her inner demons. Hans would be elated at first, but then he would quickly realize that he has a long way to go to earn Elsa's trust. He would need to prove that he is a changed man, and the kiss would be the start of that journey of proving himself.
She could interact with Daenerys Targaryen. They are both strong female characters with special powers. They might initially be rivals, both vying for power and influence in Westeros, but then realize they can achieve more by working together. For instance, Elsa could use her ice powers to cool down the hot deserts that Daenerys has to cross with her armies.
Anna would be a likely candidate to rescue Elsa. She loves her sister deeply and has shown great courage in the past. She would stop at nothing to find and free Elsa from Hans.
Anna could be the one to save Elsa. After all, she has proven herself brave and determined in the original story. She would stop at nothing to rescue her sister from Hans's clutches.
She might face the challenge of controlling her powers during labor. Since her emotions are closely tied to her ice powers, the pain and stress of giving birth could cause unexpected ice surges. Also, she has to deal with the public perception. As a queen, her giving birth is a big event for the kingdom, and there could be a lot of pressure on her.
Anna would surely be the first to help Elsa. Anna has always been there for her sister, and when Elsa gets sick, she would do everything she could, like bringing her warm soups and staying by her side to keep her company.