There are three pillars or conditions for the marriage contract in Islam:
1. Both parties should be free of any obstacles that might prevent the marriage from being valid, such as their being mahrams of one another (i.e., close relatives who are permanently forbidden to marry),
whether this relationship is through blood ties or through breastfeeding (radaa’) etc., or where the man is a kaafir (non-Muslim) and the woman is a Muslim, and so on.
2. There should be an offer or proposal (eejaab) from the walee or the person who is acting in his place, who should say to the groom “I marry so-and-so to you” or similar words.
3. There should be an expression of acceptance (qabool) on the part of the groom or whoever is acting in his place, who should say, “I accept,” or similar words.
The conditions of a proper nikaah (marriage contract) are as follows:
1. Both the bride and groom should be clearly identified, whether by stating their names or describing them, etc.
2. Both the bride and groom should be pleased with one another, because the Prophet (peace and blessings of Allaah be upon him) said: “No previously-married woman (widow or divorcee) may be married until she has been asked about her wishes (i.e., she should state clearly her wishes), and no virgin should be married until her permission has been asked (i.e., until she has agreed either in words or by remaining silent).” They asked, “O Messenger of Allaah, how is her permission given (because she will feel very shy)?” He said: “By her silence.” (Reported by al-Bukhaari, 4741)
3. The one who does the contract on the woman’s behalf should be her walee, as Allaah addressed the walees with regard to marriage (interpretation of the meaning): “And marry those among you who are single…” [al-Noor 24:32] and because the Prophet (peace and blessings of Allaah be upon him) said: “Any woman who marries without the permission of her walee, her marriage is invalid, her marriage is invalid, her marriage is invalid.” (Reported by al-Tirmidhi, 1021 and others; it is a saheeh hadeeth)
4. The marriage contract must be witnessed, as the Prophet (peace and blessings of Allaah be upon him) said: “There is no marriage contract except with a walee and two witnesses.” (Reported by al-Tabaraani; see also Saheeh al-Jaami’, 7558)
5. It is also important that the marriage be announced, as the Prophet (peace and blessings of Allaah be upon him) said: “Announce marriages.” (Reported by Imaam Ahmad; classed as hasan in Saheeh al-Jaami’, 1027)
The conditions of the walee are as follows:
1. He should be of sound mind
2. He should be an adult
3. He should be free (not a slave)
4. He should be of the same religion as the bride. A kaafir cannot be the walee of a Muslim, male or female, and a Muslim cannot be the walee of a kaafir, male or female, but a kaafir can be the walee of a kaafir woman for marriage purposes, even if they are of different religions. An apostate (one who has left Islam) cannot be a walee for anybody.
5. He should be of good character (‘adaalah – includes piety, attitude, conduct, etc.), as opposed to being corrupt. This is a condition laid down by some scholars, although some of them regard the outward appearance of good character as being sufficient, and some say that it is enough if he is judged as being able to pay proper attention to the interests of the woman for whom he is acting as walee in the matter of her marriage.
6. He should be male, as the Prophet (peace and blessings of Allaah be upon him) said: “No woman may conduct the marriage contract of another woman, and no woman can conduct the marriage contract on behalf of her own self, because the zaaniyah (fornicatress, adulteress) is the one who arranges things on her own behalf.” (Reported by Ibn Maajah, 1782; see also Saheeh al-Jaami’, 7298)
7. He should be wise and mature (rushd), which means being able to understand matters of compatibility and the interests of marriage.
More Info about walees in Islam:
The fuqahaa’ put possible walees in a certain order, and a walee who is more closely-related should not be ignored unless there is no such person or the relatives do not meet the specified conditions. A woman’s walee is her father, then whoever her father may have appointed before his death, then her paternal grandfather or great-grandfather, then her son, then her grandfathers sons or grandsons, then her brother through both parents (full brother), then her brother through her father, then the sons of her brother through both parents, then the sons of her brother through her father, then her uncle (her father’s brother through both parents), then her father’s brother through the father, then the sons of her father’s brother though both parents, then the sons of her father’s brother through the father, then whoever is more closely related, and so on – as is the case with inheritance. The Muslim leader (or his deputy, such as a qaadi or judge) is the walee for any woman who does not have a walee of her own.
If a Non Muslim lady reverts to Islam, to , none of her kaafir(non-Muslim) family members can act as a guardian (wali) of her interests; no kaafir can act in this capacity over a Muslim. If there is a Muslim with some authority in the area over the affairs of the Muslim community, then he can act in this capacity, based on the Prophet’s (peace be upon him) hadeeth:
“No marriage contract can be concluded without the presence of a Wali. A Sultan (authority figure) can act as a Wali for those without one.” (see Ibn Majah and Imam Ahmad, Hadith number 1880; also in Salih al-Jaami’, hadeeth number 7556.)
If there is no authoritative Muslim person, then one should refer to the community Muslim leader or any Muslim who is just (‘aadil), respected, and of high character, such as the director of the Islamic center or its imam, to conclude the marriage contract of the sister, with her consent.