While she may know that she is not the biological child of her parents (it may be obvious, depending on their respective ethnicities), she's been with them and their family for five years, while she is unlikely to remember more than tiny traces of her birth mother. Returning her means uprooting her from her entire life as she knows it; it's quite likely she'll come to regard her birth mother as "that woman who took me away from my family," IMO. No matter how kind she is, that's how it's going to seem, especially if she's going away to a whole other country. This seems like it would create not one but two broken families.