I do think Jack took the shirts so he would have something of Ennis'. Not that it is the same, but my oldest sister died while I was a Sophmore in college. I kept alot of her clothes for the simple fact that I wanted a visible physical piece of her. I could take them out and think of her. My little boy will smell something of mine and look at me with the most unbelievable smille and say "It smells like Mommie." Melts my heart. Sents can trigger powerful memories. I smelled a lotion the other day that made me think of my Nana when I was 7 or 8 years old. That was a long time ago, but it was like I was standing in her living room.
I think that Jack knew then that he was in love and I also think that when he said he was going to Lightning Flat to help his folks out through the winter that would be his only connection to Ennis and at least he would have something to help get through the winter.
I understand completely what you mean, I have been there too......
I also lost my eldest sister, aged 22, back in 1971, and I remember just wanting to smell her clothes....I would feel that she was there with me, instantly...it was something tangible......even years later, i would hold her clothes to try to get some scent of her.
I also have a bottle of the same perfume that I wore when I was 17 when i first met my husband, and as soon as I spray it on, I feel 17 again, and I am back in 1968.
Yes, the power of smell is is wonderful, and I am sure Jack held those shirts many many times to feel Ennis near him.....when I picture him doing that, I am filled with sadness, but also pleased that he had something to cling on to.