Bravo, Hamalasyno, to you and GD's parents for accepting her sexual orientation so easily. I'm glad she felt comfortable enough to come out to all of you. I agree with PPs (previous posters) that the opinion of a great aunt probably won't mean much to her, especially one who is only related by marriage.
Since you and DB (dear brother) are "very close," though, I imagine that you see/talk to him and SIL often. If so, perhaps you're worried about what she might say to you about GD. I agree w/ the advice to express your love and support for GD and then change the subject. If SIL persists, perhaps let her know her comments are hurtful, as one PP suggested, and, again, and ask her not to bring the issue up anymore. Hopefully, that will be enough to bring the problem, if there is one, to an end.
But maybe you're afraid that she'll make cruel comments to/in front of GD or her parents? Hopefully, she'll know better. But if she does make harsh remarks and you're there, once again, I think you should express your support for GD. Beyond that, no doubt, GD and/or her parents can speak for themselves. If it becomes intolerable, you may have to accept that they cut her out of their lives. But I hope it doesn't come to that.
Perhaps, however, SIL is only concerned with the sexual orientation of her own DC and GC, and won't comment on your GD's sexuality. Also, hopefully, DB will prevail upon her to hold her tongue about homosexuality around you and yours.
But if he doesn't, or if SIL becomes intolerable regardless, you, yourself, may have to push her away and continue your relationship w/ DB separately, if that's possible. Surely, you don't like that idea, and I don't blame you. But your "job" here, IMO, is to support GD, even if you, eventually, have to distance SIL. Once more, of course, I hope it doesn't come to that and SIL behaves reasonably and kindly.
Either way, please let us know how it goes.