We've discovered from Aatrox's VO that Morgana is somehow a Darkin who chose to stay in Runterra. We can assume if they're keeping the sister bond between Kayle and Morgana, that Kayle's sister became a host for Morgana, either willingly or otherwise. We also discover that Kayle became an Ascended, or is partially Ascended, which immediately lands her in Shurima.
Then we recall Varus' flavor story. It involves him searching Shurima for his missing sister, who he last remembers being trapped in a tomb. We can therefore assume that Morgana is going to be that sister he is looking for, as tying Kayle and thus Morgana to Shurima brings some of these pieces together.