Hello Ruben
If you start the sapinst without installing the database at one stage the installer will stop/fail with the message that database software is not installed. This will happen if the installer is not taking care of the database installation. sapinst takes care of the MaxDB systems but I don't expect a similar behavior in case of Sybase. In that case you need to install the Sybase ASE database software using the setup.bin file. In my previous reply I have said that "Most likely the ASE installation will be done using the Sybase installer". Here the Sybase installer is the setup.bin file and you will open a different session and run the ASE installation and continue with the sapinst.
Regards
RB