True, the LW cannot force her parents to accept her sexual orientation or her relationship, and it sounds like she understands that. However, she does not have to accept their order not to tell her brother either that she is gay or that she is in a relationship. Further, if her parents refuse to acknowledge that her girlfriend exists, she has to "accept" it in the sense that she can't force them to do otherwise, but I think she is right to refuse to see them under such conditions--if there is an event to which the SOs and spouses of family members are invited or would normally be included, she can refuse to come if her SO is not permitted to attend.
People who do not condone homosexuality are not required to be accepting and tolerant, but they are required to live with the consequences of being intolerant, which may include the loss of relationships with their homosexual family members. If you love your child or sibling, but refuse to accept that he or she is gay and refuse to acknowledge their committed relationships and partners, don't be surprised if your relationship with that person suffers. (I might add, this is equally true if you refuse to accept that your child or sibling or whatever is dating or marrying someone of another race, religion, class, etc. This situation is not unique to homosexuality.)