She doesn't need to let Kristina know she is being blackmailed to keep Kristina's secret. However, there is NO reason for her not to sit down with Kristina and tell her she knows that Kristina messed with the brakes that caused the accident that injured Rick and Elizabeth!
By now she should have sat down with Kristina and let Kristina tell her what was going on in her mind and for the two to agree to Kristina getting therapy.
Too much time has passed for it to be "the time wasn't right."
There is no plot reason not to have this conversation. In fact it is totally against character for Alexis not to have the conversation and there would still be the drama of Kristina not knowing her actions caused her mother to be blackmailed.
Karen L.
