Youve got it backwards.
Rachel HAD to become the rising star because Tim was caught out. The Garda investigation on his activities was going on long before the media was reporting on it. When the Allens knew the game was up with Tim they moved to push Rachel into prime position because they knew Darina would stay with him and thus be no good to them for tv etc for some time. So Rachel was on the up and up before we all knew about Tim, but not before the Allens did.
You'd swear some people were describing the plot to some daytime soap-opera. I'm not sure what inspires such apparent infatuation with this family.