I don't see Jack thinking about death and making out a will, either. However, Lureen being the methodical person who looks after every penny, would surely have done it to protect young Bobby. And she probably took care of all the detail for Jack. He probably had a life insurance policy, with Bobby as the beneficiary. As would Lureen.
I can see a conversation, casual and family in nature, where Lureen simply asked Jack about it, and he would reply that his favorite place is Brokeback Mountain, that's where he'd like to spend eternity. And Lureen would say that's an awfully long way away. To that Jack would reply, "Not if you have me cremated." So the conversation probably stuck in her mind when the occasion called for it.
If she had anything to do with his death, ALL his ashes would be gone from Texas. She was a victim in a relationship that, like Princess Diana said, had one too many people in it. She couldn't fix it so she gotta stand it, the best she could. She couldn't part with all of him, so she kept the half of the ashes that she had. The tears that she shed in the phone conversation, could be also her realization that she should have sent all of it to Brokeback, because that's where his heart and soul was all these years.