I am very sorry to read your post and I assure you of my prayers for your aunt as well as for you and your family. It is sad, of course, when events like this happen within families and I am sure it was very upsetting to learn of all of this a month after her passing.
There are a variety of distinct issues that you raise and let us try to address them point by point.
While we all should pray for the grace of a provided for death, with the help of confession, anointing and viaticum together with the prayers of the Church to accompany us to eternity, it does not always happen for a variety of reasons. Your aunt was the spiritual child of the Blessed Virgin, as we all are by the gracious gift of Jesus, and we pray that Our Lady and Saint Joseph, the patron of a happy death, and your aunt’s guardian angel accompanied your aunt from this life. And so also we pray for the happy repose of her soul.
There is no time limit for a Funeral Mass – so it is not too late, if the person who has your aunt’s cremains would be willing to cooperate. In the end, though, it does not require the presence of a body/cremains or the cooperation of other family members. You can arrange this yourself, with or without other members of the family participating. You could arrange for a Funeral Mass in the absence of a body. This would give your aunt the funeral rites of the Church, which she was denied after her passing. And the the Church’s liturgy is also a help and a comfort for those who are grieving for their loved ones, so it would be a help also for you.
Arranging for what can be arranged under the circumstances would be an act of charity, not only for the soul of your aunt but also for the living left behind…especially if she had friends or fellow parishioners who would wish to participate – the prayers are useful to the soul of the departed and also for the healing of the living left behind.
I would also add that the disposition of the body does not affect the condition of the soul by which I mean that the soul of your aunt has not been made to suffer by the choice to cremate her, which may or may not have been in accord with her own wishes.
The Church allows cremation but states that the cremains are not to be scattered, summarily disposed of, nor are they to be kept in one’s home…they should be buried or entombed in a columbarium, and normally this would be done by a priest or deacon who uses the prescribed rite so that the cremains are either in a grave or niche that is blessed.
Even though the body has been reduced to ash through the cremation process, it is this matter which will be re-constituted at the moment of the Resurrection of the Body and so the dispositions for cremains are governed by the same respect and reverence that are mandated to be shown to the body itself.
However, in the situation you describe, it does not sound as though you have any control over the disposition of the cremains. Perhaps the person who has possession of them is not aware of the Church’s mind on this matter? Would this person be open to visiting with a priest about the situation? One presumes something must happen to the cremains at some point.
Relative to this last point, and also concerning the celebration of a Memorial Mass/Funeral Mass in the absence of the body, as well as having Masses offered for the happy repose of the soul of your aunt, and also so that you can be pastorally accompanied in this very difficult moment and helped to do what is possible in these circumstances, I would advise you to contact your parish priest or the parish priest of your aunt and to talk with him. He has more than likely experienced this in the course of his ministry and can hopefully offer you more concrete help than is possible to me in this type of forum.
Sadly, as with the victims of 9/11 or the Indonesian airline disaster, sometimes there are no remains available to be retrieved to bury/entomb and this can be very upsetting to loved ones. As a purely pastoral suggestion, should this be what ends up in this case, perhaps you could establish a memorial of some sort. Shrines and monasteries will sometimes offer various opportunities to have a memorial inscribed in memory of a loved one and this would at least memorialize your aunt in a sacred place, even if her remains are not there.
I am so very sorry for your loss and once again assure you of my prayers.