Sivir Theory: what even is spell shield

So sivir has one ability, that actually makes no sense in lore. Her Spell shield. And she is unique in that way, every champion has some explanation for where their abilities come from, sivir doesn't.

So here's my theory for what it is.

Starting off, we know that sivir and azir are directly related. Second, we don't know how sivir even exist considering that xerath kill everyone in the capital, azir's wife and children included third, it is implied that magic can be passed down due to annie's origin story fourth, thanks to sylas we know that magic can be used to interact with magic

So my theory, if you haven't figured it out yet is this. One of azir's family members was born with the ability to deny magic. They most likely didn't know they had this ability until the day that xerath performed the ascension ritual. At that moment I suspect that whoever it was in azir's family who had that ability activated it unconsciously to protect themselves.

This person, who could be anyone from his daughter, to his son to even his wife or unborn child. Would wander into the shuriman deserts after the ascension and find themselves among a tribe. yada yada the magic gets passed down and we eventually arrive at sivir.