Selecting record subsets for a calculation

I'm doing a study of height & weights - and I'm trying to set up a calculation that compares differences of values in selected fields just among subsets of twins (I have 7 sets of twins in data set of 1100 records so far). I created separate fields for: a) Twin set number; twin set letter (a, : to specify each twin within the twin set, and finally 3) a concatenated field of twin set # &letter (so concant. field would have values like 1a, 1b, 2a, 2b, etc. for each individual twin). How do a set up the syntax for the case or whatever calculation to look for twins in same set - and subtract, for example, the weight fields between 1a & 1b; 2a & 2b, etc. - thus only on members within the twin sets? Please send sample syntax. Thanks.