Is he sinful who showed his aunt how to send her pictures to her fiancé and his sister?

The Sharia has permitted the suitor to see the woman he intends to propose to, due to the overriding benefit that results from it. The principle is that he should see her in person, not just her picture, because the truth might not be clear from a picture.

If this is not possible, it is permissible for him to look at her picture to get an initial impression, based on which he can decide whether to approve or reject. This is contingent on the following conditions: that he is serious about the proposal, that the picture shows the woman's face and hands and what is customarily uncovered in front of mahrams, that the viewing takes place through a trustworthy intermediary, and that the suitor does not keep the picture or allow anyone else to see it.

If these conditions are met, then there is no harm in it. If the proposal is called off, he must return the picture to its owners or to the intermediary.