E.B.
My dad was a pastor for nearly 50 years. He would conduct funeral services in which he would not shy away from declaring the Word of God that he believed so strongly in, but he would not get into social situations or disparage anyone at the funeral. He would talk about peace, forgiveness, comfort, heaven, but he would be respectful of the variety of people who would be at the funeral: truth with sensitivity. I remember one funeral where the deceased was a grandparent of a very traditional family. My dad was informed that one grandson was traveling from a significant distance, and would be performing a special tribute and that it might be unusual. The family told my dad a little about the grandson's beliefs and it was clear they didn't agree with the beliefs but that he was still their family member and he was going to be there because he loved his grandparent. The grandson was introduced by my dad, and proceeded to do some kind of ceremony involving placing stones in a circle, crumbling herbs, lighting a feather on fire, while reciting a chant of some kind. The grandson explained to the family that by lighting the feather, his grandparent's soul would be free to join the smoke and therefore join the universe, or something to that effect. Of course, my dad did not agree, but he stood quietly by and his words immediately following this feather ceremony were to the effect of "thank you, you obviously loved your grandparent very much", and went on with the funeral. My dad demonstrated a respectful stance (not rolling his eyes, or appearing bored, or anything else) and therefore set the tone for anyone else who might be inclined to be dismissive of such a ceremony at a traditional church funeral.
My dad was informed in advance. I think that many ministers know how to be respectful while not turning their backs on what they hold dear. I think if you inform your pastor that your brother will be sharing something personal about his grandmother's acceptance of him despite any differences they may have had, and that your family does not want the pastor's words to be a sermon on any social situations (whether it's homosexuality, the presidential election, responsible home ownership or how to adopt a dog from a shelter), but to focus on your grandmother's life, her kindness, her faithfulness, and her love. Ask him to set the tone for the service and to ensure that all are welcome to celebrate your grandmother's life. Your grandmother's pastor is free to share your grandmother's beliefs, and he probably will read some words of Scripture, but they don't have to refer to homosexuality or Noah's Ark or Adam and Eve or anything except comfort for your family knowing that your grandmother was faithful and loving. If he chooses to use this time of mourning as a chance to preach against homosexuality, he is a discredit to pastors everywhere who are thoughtful, dedicated, sensitive, bold, and appropriate.
I know that my dad would have appreciated knowing what your brother was going to share, and that he would have demonstrated genuine love, inspired the rest of the family to be respectful, and he would have shared his beliefs while not undermining anyone else's. I would also encourage your brother to not use this time as a chance to preach a bit of a sermon of his own. It's not the time to bring up Matthew Shepard or bigotry. He can say something to the effect of "when I told Grandma that I was gay, she hugged me, she told me how much she loved me, she told me that some people would not be accepting of me, but she demonstrated that she would always treat me with kindness. I will always appreciate that and hope to live up to her standards, to treat everyone with kindness and to honor her memory by sharing the kind of love she had for all of humanity."
I'm sorry for your family's loss.