I don't believe it's a facade. Actually I think she's one of the most honest characters in a while. Remember that page, when she was introduced to her new family? She was with her mother, all in bruises with dead eyes on her face? I assume her previous family was a life of physical and emotional abuse. And after that hell she was introduced to her new family. A family that wasn't all sweet and shiny, yes, but she could see that they were good people and it was a new warmth that she hasn't previously felt. It may seem weird from a normal person's perspective, but to her...she was just glad.
thats just mere speculation the manga doesnt show anything to sugest it maybe she was just sad for his fathers dead and thas why she has that dead expression youre overthinking it this one shot is pretty bad indeed maybe with a continuation that settle all that things you especulate this could pass from a 4 to a 6 max but right now the previous comment just hit the nail the black-hair girl didnt make anything to make me think she s not a worthless piece of trash she didnt apologize for ruining the dress she didnt apologize for being a bitch to her sister she didnt became a better daughter the cute blush didnt make for me either