More than being accepted, any queer person loves being understood especially by their dearest people like siblings with whom they share almost everything and spend days and nights together with the bonds of brotherhood and sisterhood. If you are blessed (definitely, a blessing) with a sibling who is queer, then it becomes a little hard for you to perceive and understand, specifically if you are straight. Regardless of being open or closeted to society, your queer sibling always comes out clean and never wants to hide one's sexuality and gender identity. But the question is, are you the person who truly supports your sibling and stands as a protective shield against judgmental criticism?
Unfortunately, many queer people can't be able to find that comfort zone with their brothers and sisters because of too many stereotypical societal norms. Even though if one comes out as queer, many heterosexual siblings find it difficult to digest the fact and accept it, let alone provide the support. It is distressing for every queer person out there who lacks support from one's dearest brother, sisters, and cousins because talking to parents and elders is more difficult than talking to your siblings. But if a queer teenager or an adult doesn't have a chance to anyone that is closely related to him/her/them, then that particular scenario is torturous to imagine.
If your brother or sister finds oneself as queer in terms of gender identity other than male and female and sexual orientation other than heterosexuality, then you must understand the broadening spectrum of the world around you and try to comprehend everything about your queer sibling. And this surely doesn't happen in a single day. You need to face hurdles emotionally and also face society psychologically to lend your unconditional love and support for someone who looks up to you to understand better than anyone else in the world.
Try to develop your sibling to feel comfortable in your presence even though you both look at things differently, especially in terms of interests and preferences.
Take the queerphobia out of you and all the theories that were forced to believe since your childhood by typical society because all of that is the narrow-mindedness and nothing else.
Don't sound as if you are granting some favor in terms of supporting your gay brother or lesbian sister. After all, you remain his/her brother or sister and you need to embrace that sibling factor.
It is not your responsibility to stand with your queer sibling but in the role of a brother or a sister, you need to perceive the person as a fellow human being with whom you grew and brought up together.
Maintain secrecy and allow privacy to your queer siblings. This shows the mark of respect and giving space to your loved ones.