Everybody knew he did it.

The other families would have no need to 'say anything' to the law, even they knew it was Michael Corleone who committed the murders of Sollozzo/McClusky. Trouble is, once he was dead McClusky's ties to organized crime was revealed via the Corleone's people in the newspaper business.

Yes, Michael may have been in trouble legally upon his return from Sicily, but it was now up to Vito to use HIS connections (the judges in his pockets, etc...) to clear Michael of these 'false charges'. Which of course were not really false at all. I'm sure more drawn out detail is made of this in the novel, but that's about it in a nutshell.

However as Vito later lamented...there went his hopes for a 'Senator Corleone', or a 'Governor Corleone'.

Incidentally, helpful as it is, one need not have to 'read the book' in order to to comprehend all this over time.

AppleOnYa
Non-reader of the book