If you don’t work out a different routine and stand your ground, you are going to emotionally burst! You’ve had a lot to deal with and I’m sorry for your losses.
Talk to your husband, sort out a compromise, and get him to tell his parents. You need a break from entertaining people when their visits coincide with significant dates. Not for evermore, but just for a year or two. You can welcome them at other times of the year but for now, you need breathing spaces.
Having said that, bereavement mourning is not linear, it comes in waves, so if it is too overwhelming why not contact Cruse the bereavement charity for a bit of counselling?
Also, whilst PIL are visiting, could you leave baby with them and DH and carve out some quiet time away from the house, the park, an open church, a country path, where you can reflect and remember your loved ones?
If your PIL and DH scoff at your feelings and are unwilling to adjust, that’s a whole different story.