Just my personal opinion, I feel as though the younger of the 2 is a better example of Black Pastel. I think the "cheerios" in her pattern are a better Black Pastel trait than the wonky alien heads, so there is an argument for keeping her.
But if her dad looks exactly the same, and his pattern is like hers, it makes sense to let her go. Her half sister is still Black Pastel and is closer to breeding age/size so she should work in future plans, and get there sooner.