I agree with several PPs. It seems like your MIL may have form for not respecting your BIL’s boundaries (nor do you, if you’re Googling to find his workplace). Because for some reason, your FIL knew where BIL lived and had met BIL’s girlfriend a few times and still didn’t mention any of that to MIL... or you. Time to really think about why he wouldn’t share that information.
It’s not untoward at all to want the comfort of your partner when your parent is passing, and FIL had previously met her so I think it was fine. BIL could have brought some other close friend you had never met as moral support - would that have been better or the same? If it would have been better, then that blows the MIL hating strangers in the house bit out of the water, and your MIL is upset about her son’s girlfriend for some reason (the apron strings you mentioned?). Regardless, obviously BIL wanted his partner’s moral support, so it is what it is.
But the fact is if he was not hiding info from his father (your FIL) but didn’t share it with the rest of you (you, your husband, MIL), then he must believe you won’t respect his boundaries. And MIL’s belief that he would give up his whole independent adult life to move back home after FIL’s passing, when you and her other son live right next door, seems like proof of that lack of respect for his boundaries.
I’m terribly sorry that you’ve lost your FIL and I’m sure you, your husband, MIL, and BIL are suffering. Grief is terrible
But don’t freeze your grieving BIL out over unintentional hurt feelings. Communicate, and maybe try to get to know his partner now that the door is open; even if you think his timing was off, he’s introduced her now, and your family can only benefit by being welcoming.