If you can try to be patient. Nothing lasts forever and over time (only a few months since MIL died, so very raw for them), you may well find that his visits start to lessen. I had something similar with my DH, when a member of his family was ill and he spent a lot of time dealing with them, while their immediate family seemed to do very little. It annoyed me at the time, but I kept quiet, as every time I mentioned it he was defensive, but over time when things settled down, the visits became further and further apart.
The Republic of Ireland and their tensions with migrants.