All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
It is disliked to condition the vow upon the achievement of a given matter. It is confirmed that Ibn 'Umar narrated that the Prophet forbade this as he said: "It does not repel anything but it only causes the stingy person to spend his money." [Al-Bukhari and Muslim] Therefore, the least thing that could be said about this kind of vow is that it is disliked. For this reason, a Muslim should not make this kind of vow. However, if one vowed to obey Allaah, he is obliged to fulfil the vow, as the Prophet said: "Whoever has made a vow to obey Allaah, he should comply with it." [Al-Bukhari] As regards an unconditional vow that one vows initially to fulfil with the object of getting nearer to Allaah, without seeking the compensation, then this is permissible and desirable. Imaam At-Tabari stated commenting on the verse in which Allaah says (which means): {They [are those who] fulfil [their] vows…}[Quran 76:7], this means: "They used to vow to obey Allaah, for instance to perform the prayer, to give alms, to perform Hajj and 'Umrah."
Finally, since the vow that your family is making appears to be from the first type, which is the disliked or forbidden one, then we advise you clarify to them that the Prophet forbade us from doing so.
Allaah knows best.