Yes, but in the new lore, they don't know each other, and Riot really sterilized their rivalry. Now, Malzahar just opened a void hole in Kassadin's village for the hell of it, which is how Kai'Sa got trapped for 10 years. Since Kassadin's return, he's upset and seeking him out, but there is no personal relationship between them.
Their relationship boils down to this now:
https://i.4pcdn.org/tv/1527848162920.jpg
And on that note, neither Kai'Sa or Kassadin in any of their lore or lines acknowledge each other as parent and child, which is another bizarre sterilization on Riot's part.