Clare,

Firstly, very sorry to hear of your mother's passing. It must have been awful for you, particularly during this time, and it's beyond dreadful that your mother in law has said absolutely nothing to you. Don't allow any feeling of you not being entitled to feel very angry about this to take over. Our rightful feelings can often be invalidated by individuals, or even just society's lurch towards ever increasing narcissism and lack of empathy! You are absolutely right to be seriously angry! 
 

Secondly, as you can see from the comments you are not alone in this regard. That's not intended to make it alright or remove any of your pain but there are loads of very empathetic people out there who are causing extra pain and suffering to a great many! 
 

It's too convenient to suggest that one should just accept that certain people lack any empathy and therefore aren't necessarily intentionally hurting you but it's serious selfishness personified and should be called out for what it is!  
 

Last year my uncle died suddenly after apparently being fit and healthy 1 month before. My girlfriend's mother said absolutely nothing to me despite being in a group chat and being aware that he'd died. That really hurt me.

 This year in May my grandad died suddenly of a brain aneurysm. Again absolutely zero contact or condolence, despite being aware of the fact. And neither did my girlfriend's father or brother contact me. Yet just a few days before he died, her mother could privately message to inform me that they'd sent me a lockdown parcel and it would be arriving that day (the narcissism is incredible!). So she could get in touch to tell me essentially how wonderful she was in having sent me a gift (couldn't even wait for me to receive and reply my thanks!) but couldn't get in touch when my grandad died suddenly!!!
My GF and I aren't married obviously but had been together for 3 years and, as I say, had a group chat with her family, spent several Christmasses with them, holidays/ trips/ outings together etc

My girlfriend couldn't understand fully why I thought it was such an issue of upset and I've left the relationship. There were other factors but I couldn't take any more of their inherent selfishness and lack of empathy for others - I told my gf I couldn't possibly bring up children in that environment if she wasn't willing to recognise the major fundament issues with it! 
 

Unfortunately this kind of poisonous lack of empathy seems to becoming more common now. My advice (not that you need any) would be never to reward this behaviour - treat it with the contempt it deserves! 

So sad

So sad