Op - I think some of the advice you are getting re falling out with your mother over this is not good.
I mentioned up thread about my cousin's wedding where one out of his 3 cousins was invited with dp and adult dc. This caused major upset to my mother as the wedding was huge and she is extremely close to her db, who "approved" the guest list. I was one of the 2 not invited. To be honest, I was just as pleased but ended up very annoyed as it upset my mother so much. She would have been fine with none of the cousins being invited, it was the arbitrary selection that upset her.
She brought it up with her brother, who did the whole "nothing to do with me" bit. However, he let it slip that he had been consulted on the guest list but not mentioned the exclusion.
My dsis, who was invited, and her adult dc, were going to decline as I wasn't invited. I told them not to do that on my behalf as that would have caused more friction. I did hold my uncle responsible for the mess as he could easily have told his son to either invite all 3 or none but he was more concerned about ensuring his friends were there. However, I told my mother that it wasn't uncle's fault, that it was up to cousin who he invited etc.
The reason I did that was not because I cared about uncle/cousin but because I didn't want my mother to feel she had to choose. She adores her brother and would have been broken hearted to have a major falling out with him.
I think the best thing for you to do would be similar. Make very little of the snub. Don't make your mother feel like she needs to choose. Obviously, let dn/db organise transport for your dm and don't get into discussions about the wedding.
That way, there will be minimal fall-out. You know your dn is an asshole. You know db1 is a weak, inconsiderate person but at least you will maintain relations with your dm and she won't irrevocably break relations with db1 and dn.