Is the marriage contract valid in the presence of the guardian and two witnesses who are the wife's sister and her niece? And is the marriage valid with only the guardian, without witnesses, based on the Maliki view?

Marriage is not valid unless its conditions are met, the most important of which is the presence of witnesses. The Malikis stipulate witnesses for the validity of marriage and consider their presence at the time of the contract desirable, and at the time of consummation obligatory. So, if the marriage is entirely devoid of witnesses, it is invalid. However, if witnesses are brought in at the time of consummation, following the Maliki school of thought, it is valid. It is advisable to strive to bring witnesses to the contracting session to avoid dispute, bearing in mind that one of the conditions for witnesses is being male, so a woman's testimony for marriage is not valid according to the most preponderant opinion.