Is it okay to not have a relationship with a sibling?
No, it is not "okay" to not have a relationship with a sibling. You have not offered any special circumstances to consider, so I can only say that, as a general principle, families are expected to maintain peaceful relations and to reconcile their differences. Jesus went so far as to warn those who refused to reconcile with a brother that they risked eternal damnation:
I say to you that every one who is angry with his brother shall be liable to judgment; whoever insults his brother shall be liable to the council, and whoever says, "You fool!" shall be liable to the hell of fire. So if you are offering your gift at the altar, and there remember that your brother has something against you, leave your gift there before the altar and go; first be reconciled to your brother, and then come and offer your gift (Matt. 5:22–24).
Jesus' words here usually are interpreted to refer to fellow Christians, who are brothers in Christ. But in the first letter of St. John, the evangelist draws on an allusion to Cain and Abel, natural brothers, to make much the same point:
By this it may be seen who are the children of God, and who are the children of the devil: whoever does not do right is not of God, nor he who does not love his brother. For this is the message which you have heard from the beginning, that we should love one another, and not be like Cain who was of the evil one and murdered his brother (1 John 3:10-12).
If there are special circumstances that seem to indicate a need to distance oneself from a sibling, I recommend discussing the issues with a family counselor and/or a spiritual director. If there are lesser measures, short of shunning, that can be taken to ensure personal safety, then those lesser measures ought to be considered.