Praise be to Allah.
Our advice to parents and siblings is not to prevent sons from getting married to whomever they want, if she is religiously-committed and of good character.
They should strive to make their sons happy; marrying a son to a woman he does not like puts him in a difficult situation: either he will divorce her or he will live with her even though he does not like her and his heart is attached to another. This is harmful for him and for her, and exposes the family to disintegration.
The parents do not have the right to force their son to marry someone whom he does not like, and it is not permissible for them to cut him off if he goes against them in that. It is haraam for a believer to forsake his fellow-believer unless there is a shar’i reason for doing so.
The Prophet (peace and blessings of Allaah be upon him) said: “It is not permissible for a Muslim to forsake his brother for more than three days.” Narrated by al-Bukhaari, 6065. The sin is more serious if the forsaking for no reason involves a son, mother, brother or other relative, because in that case he is combining two sins: forsaking a Muslim and severing the ties of kinship.
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said:
The parents have no right to force their son to marry someone he does not want. If he refuses he is not being disobedient; it is like forcing him to eat something he does not want. al-Ikhtiyaaraat, p. 344
Based on this, what you did was not a sin, but you have to continue to honour your mother and uphold the ties of kinship with your sisters as much as you can. Try to reconcile with them. Go to your mother, kiss her head and hand, for she only wants you to be happy. Tell her that you are happy and ask her for her approval.
Do the same with your sisters.
If they continue to cut you off, then ask Allaah’s help and continue to honour them and uphold the ties of kinship with them. Remember the words of the Prophet (peace and blessings of Allaah be upon him): “The upholder of kinship ties is not the one who is kind to them if they are kind to him, rather the upholder of kinship ties is the one who, if his relatives cut him off, he upholds the ties of kinship with them.” Narrated by al-Bukhaari, 5991.
We ask Allaah to set your affairs straight and those of all the Muslims.
And Allaah knows best.