I think your cousin withdrew because he was tired of all the judgement. A lot of times INTJs don't mean to be jerks, it's just that we don't usually get offended and we're always surprised when a family member (Usually a SF) asks us increduously, "Does ANYONE really find what you talk about even remotely interesting?" Or a co-worker says, "Geez! This is the break room! Give it a rest!" (Even though you weren't talking to them). We're just awfully bad at small talk it seems like a waste of time!
I have an ESFP friend who is really concerned about her 5 year old's lack of interest in friends . . . he just doesn't need them and that's okay. She doesn't understand it--at all! She's always been the life of the party. I told her to stop forcing him into extroverted situations and let him follow his dreams and more than anything let him know it's okay to be him!!!
I have been tempted to withdraw many times in my life, but thankfully I can't go more than 6 months before all those darn (sweet and adorable) NFs start calling me and dragging me out for coffee/hiking etc.
I've reached a point in my life where I am tired of judging and being judged!
I'm so glad your cousin has you!