Are you adding children to your current data set? I ask because consider this case:
You start with
A1, Y1, X1
A2, Y1, X1
A5, Y3, X2
A6, Y3, X2
(two simple family structures and you assign 2 ids)
later you add this child:
A10, Y1,X2
Which existing family would this go to? Reassigning existing ID would very likely be a very poor process. But your "rule" says A10 is associated with both of the existing "families".
And you don't mention the ages of the children involved so what about:
A230, A5,X10
where your "children" are also parents?
While there many reasons to have single variables for simple code there are times when they can complicate other logic.
Note that there are some procedures actually deal with pairs such as mother/fathe. Proc Inbreed is one.
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