I've been a teacher and member of SLT and have worked in schools where I wouldn't even ask, to where I'd probably have been given the day off.
But of those schools I'd have been given the day, I've absolutely seen it snowball and the headteacher has hugely regretted it and often had to revise policies on giving time off.
Sandra is given a day off for a wedding, so Tim asks for a day because his half term holiday flights are Friday to Friday, so Joan asks to leave at lunch time on a Thursday as she has concert tickets and needs to travel there... Because you've said yes to one, you can't start saying no. On top of covering sickness, funeral leave, parental leave emergencies, cover for courses, etc, it just becomes rediculously.
As a teacher, I accepted I had no flexibility in my holidays, and in return I got all the school holidays and the flexibility to leave by 3.45 if I really needed to on occasion.
Your relative has absolutely booked a Monday wedding to save themselves money with no regard for the majority of their guests. As it happens, a Monday wedding would totally suit my working week in my current career, but for most people it will mean one or two days unpaid leave (or using up holiday days).
I've missed weddings as a teacher (friends and DHs family which he went to alone), but I'd never have had a close relative book a weekday wedding if they actually wanted me there. We just wouldn't do this in my family. And if they did, I'd just let them know that I couldn't make it - that's their problem, not mine to worry about.