The terms of the original treaty called for a Princess of Uromathia to wed the Crown Prince of Ternathia. With Janaki dead, Chava had accepted the modification of Prince of Uromathia and Crown Princess.
Now last thing I remember the situation seemed to be everyone was assuming that with Janaki's death Andrin is to marry a Prince of Uromathia, so when did that change take place?
Secondly why did Zindel agree to it? The prior situation was much better, Andrin would marry one of Chava's daughters, who would abuse Andrin and there's only a chance she'll betray them, where a son will do both of those things. As a bonus it would obviously exclude any member of Chava's family from the Imperial line of succession. And Chava would have no grounds to complain, he was getting EXACTLY what he wanted.