Lots of people would hate people turning up without an invitation and with no prior warning and expecting to stay for the weekend.
You are correct, but that's not what is happening here, it has already been agreed that SIL and family are coming and which weekend it is, which is her DH birthday weekend, so there IS prior warning, OP is worried her PIL are going to turn up unannounced, but that is able to be sorted with a phone call or a message surely, anything like this should really be planned not left to chance.
But the problem is OP knows when it is and has said doesn't want them there at all, not that she doesn't want them to turn up unannounced. I agree with the PP that that is unreasonable IMO, it's her DH birthday weekend and if he wants his family there he should be able to have them and he should be making provisions to ensure he takes care of everything, if he won't do that then she has a DH problem not an in law problem, they could even talk to them now and ask them to stay in a hotel.
Like everything in life, things would be much easier to sort if everyone communicated properly, she says she wants them to visit more often, but doesn't want them to visit for her DH birthday, yet won't call or talk to them about visiting, or alternatives that are acceptable to her? It's all very mixed messages, maybe his parent turn up unannounced because that is the only way they get to visit?