My sister, who is a believing Christian, has since a while contact with our dead mother through a so called "medium".Since I am a Muslim I don't believe in such practices but I don't know exactly how to explain her, not to continue with this.
I'd also like to know more about this subject, what is said in the Quran about this exact matter.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Your sister's claim that she has a contact with her deceased mother by means of a medium is a lie and pure charlatanism. This medium is probably from the spiritualists who use different spiritual inspirations, different factors and scientific tricks, and the use of Jinn and Satans.

The Professor Mohammed Mohammed Hussain unveiled in his book "The Modern Spirituality" (Al-Ruhiya Al-Haditha) many tricks and forgeries of these people.

Professor Ahmed Azzedine Al-Bayanuny revealed in his book 'Al-Iman Bil-Malaika' (The Belief in the Angels) the lie and charlatanism of these spiritualists. He reported a personal story that happened to him, experiments with other people, that the soul that comes is only Satan in the image of the deceased person. This Satan might be the Satan that kept company with the deceased person before his death, and therefore he answers the questions of the relatives of the dead. The jinn and the Satans have powers that humans do not possesses.

You have to thank Allah who blessed you with the bounty of Islam and explain to your sister that it is impossible for any person to bring back the soul of a dead person or get in contact with it.

The human being is constituted of body and soul. So the body or whatever remains from it, remains in the grave until everybody is resurrected on the Day of Judgement. As regards the soul, when it departs from the body, it goes to the Barzakh, and it is either enjoying the pleasures of Allah or is being tortured. It is far from being in contact with anyone who claims that he has contact with it. Allah informed us in His Book that he takes the soul of men, and He keeps back from returning those to life of whom He passed the decree of death.

Allah says: "It is Allâh Who takes away the souls at the time of their death, and those that die not during their sleep. He keeps those (souls) for which He has ordained death and sends the rest for a term appointed. Verily, in this are signs for a people who think deeply. (Az-Zumar 39:42)

For more details please refer to the book "Aalam-Al-Jinn Wal-Shayateen"

(The World of Jinn and Satans) authored by Doctor Omar Suleiman Al-Ashqar, it contains a detailed report about this subject.

He mentioned some renowned names of spiritualists who were abandoned by their satans, so they resumed their situation and became like other ordinary people, they do not communicate with the Jinn and could not bring back the soul of the dead as they are claiming.

Allah knows best.