In modern versions of MS-Project you could employ a Task Calendar to ensure that the Release milestone task was only ever scheduled on dates that are relevant. I cannot really remember the functionality present in MS-Project 98, but I don't think it included Task Calendars.
An alternative way of achieving this would be to apply a special "Release" resource to the Release milestone and set the working time for the Release resource such that they are only working on dates that are relevant to the release. That way the milestone itself can only be scheduled by MS_Project for those dates. However it will be a fully manual (though not too onerous) task to set up the special release calendar for the Release resource such that only fortnightly dates are marked as 'Working Time'. Start by making the calendar all non-working time and then set the "special" days as Working Time.
So you can then make the release milestone a predecessor of all subsequent tasks safe in the knowledge the release date will always be scheduled to the next relevant period.