I too am curious to see a wedding pic of Countess Maya, or their other sister-in-law, Juliet Fowler. But so far I could never find one. Maybe Gloria decided to wear the pearl tiara of Queen Eugenie because it was just too tempting and sumptuous and because she was becoming Fürstin right away on her wedding, not just the Hereditary Princess.
And it is not unheard of, that brides choose a tiara from their husband’s family. Countess Matilde Borromeo, eg. wore a Fürstenberg tiara, although there seems to be a Borromeo tiara (both her sisters wore the same diamond tiara at their respective weddings).
Princess Alexandra zu Sayn-Wittgenstein-Sayn wore a tiara that was also worn by her mother-in-law, the Princess zu Oettingen-Oettingen. Alexandra's family too still owns at least one tiara. And in all these three cases, the groom was either head of the house or the heir.
Of course my preferred solution to the problem is that of Princess Sophie von Isenburg: simply wear two different tiaras and dazzle them all!!!
Edit: Mathilde Borromeo did not marry the Fürstenberg heir, but the younger son.