YANBU to hope that they could put differences aside, but you would be unreasonable to try to force the issue or play peacemaker.
I no longer see or speak to my PILs. Long story still quite long, they are controlling and manipulative people and for eight years we had an up and down relationship in which I did try very hard to accept them as they were and keep the peace.
They then made a series of very hurtful comments about the babies we lost to stillbirth and prematurity and compounded the problem by trying to blame me for being difficult when DH eventually told them they had gone too far. We still tried to get along with them but things were massively strained and PILs decided that the best way to fix the problem was to pretty much stalk me, while at the same time as they were telling DH they loved me they were telling the rest of the family that they hated me and blamed me for everything. They told a lot of lies about me, followed me when I left the house, constantly rang the telephone up to 17 times a day, and finally drove me to have panic attacks. They have disowned DH in one telephone call and denied doing so in the next. They have shouted at me on the phone and in the street, demanding that I do as they say, do as I am told, give them their own way.
At the point they caused me to have the first panic attack, I cut them off. That was two years after DH told them they had to stop making cruel remarks about the lost babies and pregnancies.
I could be here all day giving examples of what they did and said, even DH has said that as a child he grew up being told that he could not ever upset his mother or she would have a mental breakdown that would be all his fault, so he must always let her have her own way. It took him a long time time to get over that. But just one example of what MIL said to me was to ask me, three days after I had given birth to our stillborn son, if it still hurts to give birth if your baby is already dead.
It's taken a long time to stop having the panic attacks and I still feel ill at the thought of coming face to face with these people. DH sees them occasionally.
I could not and would not be able to put all of that aside for the sake of a christening. We did go to a family wedding in the middle of all the trouble they were causing and that was awful. We sat at the same table and didn't speak one word to each other.
Family members have gotten involved, usually on the PILs behalf because they have told everyone their side of what went on. DH and I haven't done that. So people have tried to play peacemaker and have either been shocked when they've been set right on what happened or gone down the line of "but even so, they are DH's parents and you can't just cut them out..." One of DH's brothers took their side to the point that he disowned DH and called me a fucking evil, mad lying bitch. All because PILs told him a pack of lies and half truths about the trouble between us.
But I can and I have, and I feel so much the better for it. Those people made me ill, and I have to protect myself and my son from them and their vile behaviour.
I have a long and unanimous AIBU thread somewhere, telling me IANBU and since that point I could add that PILs have been going to our babies grave and interfering with it, taking fresh flowers we have left and throwing them away, balancing strange objects on top of the gravestone etc. I've had to report the problem to the cemetery security and they have reported it to the police, and we were forced to put a notice on the grave telling people that the police are involved and not to remove or add anything to the grave. It's bizarre, and I can hardly believe that this is what our relationship has come to because for those first eight years it was difficult at times but not always bad.
I don't know if what went on with your cousin and mother is anything like this, you'd probably remember it if it was, but sometimes putting things aside isn't possible, and your son can still have a lovely day without the people who aren't attending. Has your mother said what she would do if your cousin and aunt had accepted the invite?