Of course there would be no sin.
Are people not seeing the intent here? The OP specifically asked if they may avoid marriage until after the fertile years. If that is the way it works out then sure many people cannot find a mate until later. Or find another mate later. None of that is disputed. What is disputed is purposely avoiding it because of the intent of Children. Look, marraige is a vocation, what the OP is asking is if one can be called to a partial vocation. Marraige, but not marriage. Like a priest wanting and purposefully rigging ordainment to avoid saying mass but getting to hear confessions.
Again, “feelings” can cloud the theology here and people can get quite huffy with my sister this or my wife and I that, but what I am not seeing is anyone actually doing what the OP is asking. If your father’s sister (your aunt) never married or did marry later is not the issue what would be the issue is if for 20 or so fertile years your aunt said, “Well, I am going to get married but not until I am infertile.” That would be weird wouldn’t it?
That is the issue. To have a such a feeling of dread of children that one alters natural events in order to avoid the issue and construe marriage all together.
Just like NFP can be used inappropriately. This could as well. And in the OPs post, it would appear that the single reason to not get married is to avoid children, then when children are no longer possible get married. Now honestly I have no idea why someone would go through this and drag someone else through such an elaborate scheme. But then again, I have no idea why people use NFP with a contraceptive mentality either:shrug: But we are human, and we tend to be capable of doing some odd mental gymnastics to try to make God see it our way.
So, let’s go to your example. If your Aunt had the opportunity and want to get married, and even had a man all ready to go and picked out, but then delayed that marraige for however many years until full on menopause kicked in for the specific purpose of not having a child then, yes, there would be a problem there. But if she had married later because that is when she found “the one” but it was past child bearing years, then there would be no problem. I am all for people getting married. At any age allowed by the Church. (yes, even 14) But I am not in favor of using that sacrament to your own selfish wants mocking God. Nope, not a fan at all!