Hello Beryl, I'm really sorry that you have been ostracised, being excluded from family events because it reduces your feeling of belonging to those people that you once felt aligned to and have celebrated on many occasions before, it just isn't understandable.
Sometimes it can be only one or even two people who have the dominance to make this decision simply because of previous consequences that have happened not to agree with them.
There are a couple of ways to overcome this which you may want to do or not, if you feel as though you want to actually do this and just call into the pub, saying that you were just passing by and wanted to wish her a happy 80th birthday and then leave, or you may even be asked to stay.
The other option and only if you want to is to invite her out on another day, but this maybe difficult for you to do which I can understand and feel very sorry for you.
This has, unfortunately, happened to me in the past and it does leave us in a situation we wish we weren't in.