I guess your son is intent on playing happy families so he is bound to be upset. If Grandpa has a health reason, or any particular reason, for not going tell your son. It's up to you if you want to go or not, it is a chance to get to know your son's new family and your grandchild. Perhaps your son would like to pay your travel and accommodation!
The oldest Grandddaughter whom we have seen twice is getting married, we live in MT and they live in MI
I take it you live some distance apart (in the USA?) and you have no relationship with the GD because you have only seen her twice in all her life?
You have no real relationship with her, or fondness either, because she is someone not in your life. You don't know her.
I don't think the fact she is a step GD comes into it.
It doesn't sound like you have a closely knit family but a wedding means you have to fake your happiness and pretend you are supporting a granddaughter who is a complete stranger. You have no idea of her relationship with her fiance either. I take it he is unknown to you?
Do the grown up thing. No faux affection, no need to show up now after decades just because it's an occasion.
Lots of good will needed though...so send the happy couple (strangers to you) a card and perhaps a wedding present and wish them well.
Let's face it, you are already estranged. This is silly pressure to be there and the only person you'll know will be your son. (Do you remember what he looks like if there is distance between you, so much so that you don't know his family/daughter?)
Keep your husband company on the day....and don't allow your non-attendance to be a reason for quarreling.
Nothing is wrong, it is just something you have to agree between yourselves,
The two people concerned have to sit down and each explain to each other why one has decided to not make the journey and the other is upset at this decision. Listening and understanding is essential, shouting and temper losing is forbidden.
For example, it may be something as basic as the older man having prostate problems he has never really admitted to, and he doesn't want to risk the journey and service, in case he has an accident. The younger man may be seeing this as rejection by his father, because of past history when he was a child.
Issues like this only come out and acceptable compromises are only brokered if the people concerned can sit down and calmly explain why they think like they do, listen to the other person and then reach an agreement satisfactory for both.
Our sonless than a year ago married a girl with 7 children and he had in which make 8, The oldest Grandddaughter whom we have seen twice is getting married, we live in MT and they live in MI, we have decided I the grandmother will attend the wedding but Grandpa has deciced to stay back. Our son is pretty upset about it! please help I am losing sleep...……………………..is this wrong?