Praise be to Allah.
In order for a marriage contract to be valid it should be done by the woman’s guardian or representative. The woman does not have the right to give herself in marriage, because the Prophet (blessings and peace of Allah be upon him) said: “There is no marriage except with a guardian.” Narrated by Abu Dawood, 2085; al-Tirmidhi, 1101; Ibn Majah, 1881, from Abu Moosa al-Ash ‘ari. Classed as saheeh by al-Albani in Saheeh at-Tirmidhi.
And he (blessings and peace of Allah be upon him) said: “Any woman who gets married without the permission of her guardian, her marriage is invalid, her marriage is invalid, her marriage is invalid.”
Narrated by Ahmad, 24417; Abu Dawood, 2083; at-Tirmidhi, 1102. Classed as saheeh by al-Albani in Saheeh al-Jami‘, 2709.
The woman’s guardian is her father, then his father (i.e., paternal grandfather), then her son, then his son (i.e., grandson, if she has a son), then her (full) brother through her father and mother, then her brother through her father only, then their sons, then her paternal uncles, then their sons, then the ruler. See: al-Mughni, 9/355
This is the view of the majority of Maliki, Shafi‘i and Hanbali fuqaha, and it is the correct view that is in accordance with the texts.
The Hanafis are of the view that a marriage concluded without the woman’s guardian is valid, and some courts follow this opinion.
Because of the difference of opinion among the scholars as to whether this marriage contract is valid, if the judge issued a verdict of validity concerning this marriage, then his ruling is not to be overturned and in that case it is said to be valid, so that there will not be any confusion among people.
Ibn Qudamah (may Allah have mercy on him) said in al-Mughni (9/346), concerning a marriage contract that is done without a guardian:
If a judge has ruled that this marriage contract is valid, or if the one who did the marriage contract was a judge, it is not permissible to overturn that. This applies to all other marriages that may be regarded as invalid. And some of the scholars are of the view that it should be overturned because it is contrary to the text. But the former view is more appropriate, because this is a matter concerning which there is a difference of scholarly opinion and differences of opinion are acceptable in this case. End quote.
If your sister wants to be on the safe side, and the parents approve of the marriage, then she has to ask her husband to repeat the marriage contract with her father as guardian, so that the marriage contract will be valid beyond any shadow of doubt.
And Allah knows best.