Hongshuo x Hongyi shipper here to explain how I see Hongyi's feelings for Hongshuo.
1 Unlike Hongshuo who loves Hongyi from the beginning of the show and openly shows his care and affection for her, Hongyi's love is more subtle and develops throughout the show. At the start of the show and during the time when they lived in the the Baili household, I do think that Hongyi does view her relationship with Hongshuo as a brother/family because they are usually together most of the time and she is used to his presence. From conversations in the show, she used to follow the older brother and the father around and focused her more of her attention on them, while Hongshuo was the one following her around. Her views of him seems to be stuck as the young boy he was when they first met and the younger brother who follows her around. Then when they went to Li City, Hongyi started to noticed how mature Hongshuo was as well as noticed that him as a man instead of as boy. This is especially evident when Hongshuo gave her the obsidian stone and stood close behind her. At first she didn't notice his presence, then she became self conscious and nervous when she noticed how close together they were. At Li City, she continued to interact and observe different part of him that she didn't noticed before or had never saw before since they weren't always together like usual.Their relationship during this period in public were also different since most people didn't know Hongyi's real identity. She might have develop feelings that were more than family at this time. When she had to leave Li City, Hongxuan noticed that she was constantly talking about Hongshuo which showed how much she unconsciously was thinking more about him. This separation caused Hongyi to realize how much she missed Hongshuo as shown when she hugged him after they reunite at the Baili household and at Ye Feng's tent. Episode 24 is when she just started to accept Hongshuo's feelings for her and her feelilngs for Hongshuo as romantic. Then she remembers her memories and her feelings are on paused as she's still conflicted though she definitely still cares about him.
2 Hongyi care's a lot about family (probably since she lost her real parents) so when Hongshuo implies romantic love she tries to deflect that he's talking about sibling/family love or implies that it's just a joke.
3 Hongyi is a bit of a tsundere and her action speaks louder than her words. For example, when Hongshuo is leaving for Li City she acts nonchalant as though she doesn't care that he's leaving. However, we and Hongshuo learned that she followed him. Another example is in episode 24, where she drinked poison for Hongshuo and was willing to die with him, but won't admit to worrying about him.
People who think Hongyi only views her relationship with Hongshuo as a brother/family takes her words too seriously and upfront while not paying to her body language and actions that often contradicts her words. In the newest episodes they are in an angsty period, but I can't wait to see how their relationship will further develop and hope it will end happily.
Sorry to all Ye Feng and Hongyi shippers but here's a funny comment I saw about the two of them.
They're both adult age and probably at most ten years apart in the drama but visually they do kinda appear like this.