I think there is a very real possiblity that while the DB isn't ashamed at all that his Dsis is gay, he is very ashamed his mother is so homophobic wants to hide that from his future in-laws and friends. I can't imagine many people (outside the OP's very oddly religious mother) , would really mind or be bothered at all beyond "oh, that's lateblooming's girlfriend, she seems nice." (although that might take some attention away from the happy couple so there's an argument for the girlfriend meeting the family first so it's ot the focus of the day for the grooms family!). However, if his mum makes a scene, that would be hideous for the couple. If they can't ensure that both his mum and his dsis's DP can be in the same room without his mum shouting off, then someone has to not come.
It's a much bigger issue to not invite your mum than to not invite your sister's (not married) partner. Just saying "you can come or not" to the mum won't be enough if she possibly might come and then cause upset and ruin the day if the OP's gf is there, and this isn't about the OP and her DP, it's not about the OP and her Mum, it's not about the DB and his mum, it's about the DB and his future DW. It's about that couple and I can completely see they would want to avoid anything that will spoil their wedding day, even if it involves pandering to his mother.
OP what you have done, coming out to the extended family, stopping your DP being a secret, is the best course of action all round. If you now can start arranging to meet other family members with your DP, perhaps invite some over to yours at some point - to further remove from the wedding situation the 'novelty' of your DP. (Don't ask your mum at the same time, you dont need her permission to cal your Aunt and ask her for lunch or your cousins!)
Ultimately, you might not be able to have a relationsihp with your mum. 