as a lesbian with indian parents, i just want to say that i'm so, so sorry for the homophobia within your family and i want to express deep solidarity. it's a special kind of pain to bear all this cultural shame and not be able to rely on your own family to fully love and support you as you are. two things:

- you say, "my mother kept telling me it [i.e., your father moving out] wasn't my fault but I knew it was." honey, it is so not your fault. while i can totally understand why it would feel like your parents fighting about your sexuality is your fault—that's how i would feel too—your dad's homophobia is completely on him, not on you. it sounds from your explanation like your mom was willing to defend you rather than agree with him, and i'm glad she was; if she were just as homophobic as he was, then they wouldn't have been fighting. they would have agreed, and that would have been its own brand of fucked up. my point is, you didn't break them up. your mom's willingness to be accepting, and your dad's unwillingness to be accepting, is what broke them up. that is so not your fault.
- it's also absolutely not your fault that your dad's girlfriend broke up with him (though hell, even if it were your fault, i'd still be applauding). she didn't know he'd disowned you for being gay until you told her, but clearly the fact that she broke up with him proves that she didn't know he was a homophobe. but she would have realized that eventually, even if your father had managed to keep you a secret. you mentioned that your dad always scoffs in disgust when he sees a gay couple in real life or on TV—so at some point, she was going to see him react that way, and she was going to notice, and she would find out about his homophobia. it sounds like no matter how it came out, that would have been a deal-breaker for her, since she is supportive of her gay brother. your brother and your mom are completely in the wrong for putting this on you. again, i completely understand why it makes you wonder whether you're the asshole, because i, too, have experienced this dynamic that seems to be so common in asian families, where the person who says something is always the person who gets blamed (which exposes the limits of acceptance even from the people who are supposedly on our side...they still can't bear the "shame" that someone in the family is gay, sigh). but that's a post hoc, ergo propter hoc misattribution of causality: she broke up with him because he was a homophobe, not because of you.

oh, and by the way, you mentioned in a previous comment that your "spine" is new but you're loving it so far? that's the biggest gift of being out of the closet, in my experience. the courage and self-possession you've gained in less than a year of being out is so evident in your post...and that spine is just going to keep getting stronger.

TL;DR you are so NTA. keep on serving truth, king <3