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....

In Spanish both forms are correctly: "mis preciosas primas" and "mis primas preciosas"; In this case the adjective (precious) before or after the noun (primas) does not change the meaning of the sentence. on the other hand, "las primas preciosas de mi" is incorrect, because in Spanish prepositions are not used at the end of a sentence as in English.

Mis primas o unas primas mías.

You could said "mis primas preciosas" or "mis preciosas primas" the both phrases are fine just depends the emphasis I mean if you want to remark that are your cousins use the first phrase or for remark that they are pretty use last one