If you're still taking these, 5 and 23 for Eugénie?
5. How do they typically dress? Does their wardrobe lean more towards practicality or aesthetics?
Eugénie leans on the side of practical for Orlesian aristocracy, typically dressing in clothes better meant for riding and hunting, typically in shades of blue with silver. Two really good examples would be Duchess' base outfit in Elden Ring: Nightreign and the White Lady in Lies of P. It's all extremely well-tailored, and what she favours for her day to day wear. The cut usually gives her a slightly more androgynous look, particularly when you add her masks, but it makes her stand out among her peers and I think suits her more as an 'active' Orlesian aristocrat.
Honestly I would like to say even after her marriage, she tends to favour that style of dress in private, but for official appearances, you're looking at the more 'traditionally feminine' looks that draw a bit more from the mid-to-late 1700s (e.g. Marie Antoinette, Catherine the Great etc.) However more often than not, she will prefer to stick what is comfortable, or if it is a gown, it's more likely to still be on the more simplistic side or a riding habit that echoes a more military styling that comes into fashion with the new dynasty.
23. how would you describe their voice? can they sing?
Unfortunately, I don't quite have a specific voice in mind for her - more of a vibe, really - but the closest would be "if Freja from Overwatch was French, rather than Danish." Similar voice quality: stoic, steady, to the point, but confident in herself. She knows she's damned good at what she does and you can hear it in her voice.
As for whether she can sing - not really. She cannot quite carry a tune like others can - music was not a part of her education she particularly cared about or enjoyed. More often than not, she was content to leave that to her sisters while she focused on other things.