The sister is a layperson and a layperson (male or female) may never give the homily. Here is the relevant instruction from the General Instruction of the Roman Missal (66):
The Homily should ordinarily be given by the priest celebrant himself. He may entrust it to a concelebrating priest or occasionally, according to circumstances, to the deacon, but never to a lay person. In particular cases and for a just cause, the homily may even be given by a Bishop or a priest who is present at the celebration but cannot concelebrate (emphasis added).
Redemptionis Sacramentum (65), the most recent Vatican instruction on the liturgy, goes on to say:
It should be borne in mind that any previous norm that may have admitted non-ordained faithful to give the homily during the eucharistic celebration is to be considered abrogated by the norm of canon 767 §1. This practice is reprobated, so that it cannot be permitted to attain the force of custom.
The prohibition of the admission of laypersons to preach within the Mass applies also to seminarians, students of theological disciplines, and those who have assumed the function of those known as “pastoral assistants”; nor is there to be any exception for any other kind of layperson, or group, or community, or association.
While it would be best for the aunt of your brother’s fiancee to give her remarks at the reception that usually follows weddings, it is also possible for a layperson to give a post-Communion reflection, so long as it is not of a nature that it could be confused with the homily and so long as the homily is not impacted on its account. From Redemptionis Sacramentum (74):
If the need arises for the gathered faithful to be given instruction or testimony by a layperson in a Church concerning the Christian life, it is altogether preferable that this be done outside Mass. Nevertheless, for serious reasons it is permissible that this type of instruction or testimony be given after the Priest has proclaimed the Prayer after Communion. This should not become a regular practice, however. Furthermore, these instructions and testimony should not be of such a nature that they could be confused with the homily, nor is it permissible to dispense with the homily on their account.