What should we do regarding adult female relatives who are not committed [to Islamic dress] but have requested embroidered abayas that go over the shoulder, and we fear incurring sins by purchasing them for them, especially since we will be going down [to their location] very soon?

The abaya is not required to be a head-abaya. It is permissible to buy shoulder-abayas, provided that the woman covers her head. As for embroidery, it is an adornment that a woman is not permitted to display when going out. It is not permissible to gift or buy embroidered abayas for someone who will display them outside the home, as this constitutes aiding in what is impermissible.