Its not the secret services in the sense of CIA/NSA that are involved, but anti-corruption investigators. I couldn't locate any further details about that organization, but I would assume it resorts under the equivalent of the Justice Department. Sounds a bit familiar doesn't it? Only here the competition was targeted instead of the opposition.
A parliamentary investigation before the election was blocked so the brothers will still be able to claim that the investigators did their job as they were supposed to. I don't see them losing many votes over this one and likewise with the two coalition parties that were targeted. Their electorate does not contain any left wingers, so I expect most defections to be traded amongst the four parties (the fourth withdrew involved.
If anything comes out of this, the brothers may trade the extreme right parties for a coalition with the more centrist right wingers, but any "shift to the left" will be relative at best.
Not sure where the "old communist guys" are coming from. Most of the current crop in power have their roots in the Solidarnosc movement. One of their last major acts was an attempt to "out" all former communist associates in public life, so I don't think they are afraid of any revelations (unless of course it would be that the brothers were "ubecy")