NOTE: I haven't worked with Sybase repserver and MSSQL (as source or target) so fwiw ...
I looked through the various release bulletins for Repserver Options (15.1, 15.7.1 SP100, 15.7.1 SP120) and all I can find are references to MSSQL 2008/2008R2 ... with earlier versions of RS (eg, 15.1) not supporting MSSQL2008.
The compatibility matrix tool (http://certification.sybase.com/ucr/search.do) also does not show anything higher than 2008/2008R2; w/2003 the highest supported platform for RS 15.1.
If you're talking about 2012 as a target database you may be safe as long as 2012 can work with the drivers that come with RS 15.1. As for using 2012 as a source ... *shrug* ... I wouldn't even want to guess at this point.
Sorry, I can't tell from the various documents if Sybase does not support MSSQL 2012, or if Sybase documentation is woefully out of date.
I would have to suggest contacting either Sybase tech support or your Sybase sales rep ... or wait for someone from Sybase/Repserver engineering/support to jump on this thread.