No, I'm not against Appu at all. I'm 100% behind her knowing the truth. She deserves to know it and she has a right to know absolutely! And family should tell her and Dr.K too about it.
What I'm against is her stubbornness in dealing with it. Out of the two leads, or actually the whole family, she is supposed to be the one that understands emotions the most so she should understand how sensitive this is. She's seen how disturbed Mai was at the picnic after just hearing the name and she's seen DK and Mai actually fighting on this matter and now she's seen Kukki, who is always on her side no matter what, actually hiding something from her. So she should at least understand that the matter is sensitive and try to find out calmly instead of acting like a brat. If she has asked everyone calmly first and they refused to tell her and then she acted stubborn, I would be 100% with her.
After all, isn't this the same girl that was about to get up and leave when Sua Aai was feeding her, just because they brought up the topic of her mom.