Of course you love your dad... But your stepfather may have real and valid reasons to never share air with him again that we're never shared with you (like raping your mom, giving her an in curable disease, hurting you kids when you were too little to remember, abusing your mother, etc.). Things that aren't shared with children to protect them, and never make it into adult conversation 10-20-30 years later. But offenses that are, to some people utterly unforgivable.
Or it COULD be as 'simple', as your bio father has been off in another state for 30 years, but the moment he decides to be a part of, your stepfather gets dropped like a rock in favor for your biological father.
Or worse, a combo of the above. That your SD can't be in the same place without being at risk of killing the man (or at the very least ruining a happy event), but your biodad is the one who is the 'chosen one'.
Everyone here is a grown up. But that doesn't mean it's the grandparents who only have to act like it. Meaning just because he's your father doesn't mean SD has to a) Like him or b) even tolerate being in the same house with him as he would have had to were you still a kid. You can BOTH be grownups. He can respect you wanting to see your biodad, and you can respect that he cannot tolerate the man for whatever reason and still keep your SD in your life instead of having it be all about what you want.
If this man was good to you, and raised you, and has always been there for you... He's HURTING right now. Enough that it's divorce worthy after 25 years of marriage. Help him.
Does this mean you do double the events? Probably not. But everyone is grown now. At least go talk with him. Imagine shoe on the other foot, and it's your kids choosing someone over you. Don't go in trying to get him to see x,y,z... Go in trying to understand, and being willing to listen.
And then, maybe a compromise can be reached once you're both able to respect each others needs. Sounds like SD already is? By not coming to a gathering and ruining it. But by respecting your need to see your bio dad, SD seems to be getting shafted. Dropped.
As a kid it was my parent's responsibility to protect me. As a grownup, it's our responsibility to protect each other. When my mum is HURTING, it's not about me right then. It's about a person I live dearly, a human person with faults, in pain. I can't always make the pain better, but I can live on her and be there for her in a hard time. And STILL the bloody idiot tries to protect me. But I'm grown, now. I can handle the truth. I may not like it, but I can handle it.
Granted your SD could just be a jerk you're grateful to see the back of... But if not? Turnabout is fair play. My parents were there for me, and are... And I'm there for them.