Marriage in Islam is built upon clear conditions that ensure its validity, namely the offer and acceptance (ijab and qabul), the presence of a guardian (wali) for the woman, and two Muslim witnesses of good character. The Prophet ﷺ said: “There is no marriage without a guardian and two witnesses of good character” (Sunan al-Tirmidhi, 1101; Sunan Abu Dawud, 2085). For a Muslim woman, the guardian is normally her Muslim male relative such as a father, brother, or uncle. If she has no Muslim relatives, the role of the wali passes to the Islamic judge, or in lands where no Muslim judge exists, to the Imam of the local mosque or a trusted scholar, based on the hadith: “The Sultan (Islamic authority) is the guardian for one who has no guardian” (Sunan Abu Dawud, 2083).

In your case, since the sister’s family are not Muslim, they cannot act as her wali. Instead, an Imam or Islamic authority in your community may take on that role to safeguard the validity of the marriage. As for the witnesses, they need not be family members; rather, any two upright Muslim men can fulfill this requirement, whether they are friends, members of the mosque, or other trusted individuals in the community. Therefore, even without family approval or family witnesses, the marriage can still be valid if these requirements are met.

Parental approval is highly encouraged in Islam because of the importance of maintaining family ties and seeking parental blessings. However, if their refusal is based on reasons that are cultural, social, or ethnic rather than religious, then their disapproval does not prevent the marriage from being valid in Shariah. At the same time, it is wise to continue seeking reconciliation, making du‘a for their acceptance, and handling the matter with wisdom and patience in order to avoid ongoing family conflict.