Re: Mine crashes as well !
Most of the people who are seeing this issue have corrupt GCF files whether steam says they do or not. The most drastic way to fix this issue is to delete the .gcf's that have been downloaded for sin and re-download them through steam. If you bought the game retail, this is a fairly harmless procedure, but if you bought from steam, you unfortunately have a rather hefty download. We are currently investigating other possible solutions to this issue, but as of now re-downloading your gcf's is the supported fix.
Please make sure you close steam entirely and then delete:
The ClientRegistry.blob file in your steam folder
All SiN Episodes GCF files in your steamapps folder
Then you can restart steam and re-download SiN.