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.
If this young man is as you mentioned that he is religious and has a good moral conduct, then he should not be refused as a husband because the criteria for choosing a spouse should be religion and good moral conduct as we clarified in Fataawa 88093 and 82298.
Indeed, those who refuse to accept him because he is from a different tribe or for being from a different family status are undoubtedly wrong. Therefore, you should try to convince those who refuse to accept this marriage. If they are convinced, then praise be to Allaah, otherwise, their objection should not be taken into account. Your mother is not obliged to obey her mother (your grandmother) because her objection is not for a sound purpose. For more benefit on the limits of obedience to parents, please refer to Fatwa 84942.
Moreover, if your older brother refuses to marry you off, then your younger brother may marry you off if he is pubescent. Besides, your maternal uncle should not threaten to divorce his daughter from your brother if this marriage takes place.
Nonetheless, if you think that it is better to avoid this marriage with this young man in order to preserve the ties of kinship with your relatives, and you are safe from being harmed if you do not marry him, then it is permissible to apologize to him and Allaah may facilitate a good spouse for each one of you.
Allaah Knows best.