We don’t use “de mí” to show possession. (But we can use it to show location, example “detrás de mí” "behind me". In some regions, you may hear “detrás mío”.) We can say “mis primas” (usual) or for special emphasis, las / (alg)unas / varias (etc.) primas mías. It is a general rule that the last word in a noun phrase is considered most important, so “mis preciosas primas” takes it for granted that your cousins are cute. “Mis primas preciosas”, on the other hand, is more emphatic, and could suggest that you are making a distinction between your cousins who are cute, and those who are not. I would use the first one....