I have six brothers and I cannot imagine having some stranger pointed out to me as an additional half-brother would make him matter as much to me (for good or ill) as people I have known all their lives. Further, Rand has explicitly stated more than once that he views Tam as his father and feels little or nothing for Shaiel/Tigraine (and I would bet that most of his feelings toward her are inspired as much by the premature loss of Kari al'Thor than any genuine interest in Tigraine for her own sake), so it isn't even like he and Galad can talk about the mother they had in common. Their mutual descent from a woman who essentially abandoned both of them to be raised by the women each thinks of as his mother (and who, in each case, has a better claim to that title than Tigraine) is hardly much to base a relationship on.

Really, their presumptive future relationship as brothers-in-law is more significant than their accident of blood ties, aside from the rather interesting technicality that means Galad is just as close, biologically speaking, to Elayne's children as Gawyn - if a full-blood uncle is defined as the offspring of two of the nephew's grandparents, the forthcoming twins have four biological grandparents, Tigraine, Morgase, Tarangail & Janduin. Gawyn is the sone of Tarangail & Morgase, and Galad is the son of Tarangail & Tigraine. Each is equally close in blood-relationship to Elayne's & Rand's children.