Yikes, I can feel your raw emotion and you have my sympathies. A father-daughter relationship is very special and I'm sorry you've been deprived of yours for so many years. That, to me, is the real tragedy here.
I'm going to go against the grain here and say that, IMO, your father made his wishes clear by his actions and otherwise over the past 30 years, so his "last" wishes at the end of his life were almost a moot point. For whatever reason, he didn't want to see of mend fences with you. It's really not about your SM or your brother, it's about your father. You really need to keep that in perspective or you'll launch yourself on a downward spiral that will ulitimately only hurt you in the end. To that end, I agree you should probably skip the bday party this year as it will end up in a mess of emotions and accusations. Give yourself some time to settle down.
You should also REALLY consider talking to someone professionally about your longstanding and acute grief as a benefit to
yourself. I'll probably get flamed for this but I can say as a BTDT type of thing that someday, maybe not today or next week or next month, you can give
yourself the gift of
Forgiveness toward your father (and +/- anyone else) as it will help bring peace to YOUR life. Until then, these emotions are going to eat away at you.
I'm not saying it's easy, by any means. I've felt some of the frustrations you have. But at some point you have to let it go and get on with the rest of your life. It's tragic that this happened to you. Don't let it poison the rest of your life and other relationships.
I just want to add one thing from the perspective of a hospital nurse who sees these types of situations a lot when people are critically ill or dying. If the wishes of a patient are that they want no visitors, or no specific visitors, we honor that. If people bust in, security is called and they are escorted out. That's just the way it is. We are not privy to past events, nor do we really need to know. Our duty is to that patient at that moment in time. If your brother was in the hospital with your father, I'm sure he saw that. You said you wouldn't have done anything, but then you said you'd call SM. I suspect you also might have tried to go to the hospital as well (from the sound of your post) and that would not have been pretty. Maybe your brother was trying to spare you from that.
You also mentioned that your brother is unmarried with no children. Remember, you have your kids and presumably a partner and like most families with children, are probably pretty busy in your life. Who does/did your brother have? I think your mother is probably accurate, and naturally does not want to see her children arguing or estranged. From the sound of the relationships in your family, it seems to be your mother and your brother who've kept out of the fray as much as possible. Maybe their thinking isn't as off as you think it is.
I feel as though he made a choice between my wicked stepmother & me and choose her.
In my mind, once dad was gone, DB's loyalty should have shifted to family. In a very deliberate move
there was no chance to say goodbye while he was in the hospital, no chance at the funeral home, and no grave now to visit or pray at
I've had nearly two weeks to process it and know it was a petty, vindictive "you'll be sorry when I'm gone" kind of move, likely outdated, voiced many years ago, intended solely to hurt my aunt & I... and it did... every time I think I can put it behind me and move past this, I'm reminded and the hurt grows deeper.
My brother feels wholly justified of course and hasn't even apologized

for the hurt he participated in causing.
Going out on a limb here, not meaning to offend you but maybe offer some insight from an outsider's perspective. It's probably not what you want to hear, but I do think your thinking is off. It's almost paranoid and you seem to be projecting onto them things that may or may not be accurate. We probably all do it to a degree, especially when we're angry, but it's not fair to your brother if in fact he has a whole different perspective than you do. When things settle down you should listen to what he has to say with both an open mind and an open heart.