I think we have to be a bit cautious about the definition of "sibling". These are oral tales repeated through centuries, so the details will not necessarily be exact facts.
Ornn, Volibear, and Anivia are "siblings" in the sense that they are three kindred spirits of the Freljord, not because the same creature gave birth to all three of them. I doubt they even have parents, but instead were born from the land itself, like Maokai was.
Possibly Lissandra enchanted the bridge knowing that the spells would erode over time. She wasn't ready to turn traitor yet, so had to pretend to play her part in trapping the Watchers. By setting up the bridge to eventually fall, she gives herself plenty of time to secretly assassinate Avarosa, and maybe Serylda and the rest of the Iceborn, and to weaken the Freljord, so that when the Watchers return there is no one powerful enough to fight them again.