Sorry you're on your own dealing with your mum's illness, OP. That sounds really hard.
In the kindest way possible, though, I think yabu. Your bf's cousin has just died, he wants to be with his family, he doesn't want to risk a very long journey that could potentially make it difficult for him to get back for the funeral if there were delays etc.
Your mum is clearly very unwell and you're understandably worried, but I can see why he wouldn't think that she is at death's door if she is still going out for cigarettes etc. I'm sure that it isn't that he doesn't care, but he needs to prioritise his own family right now.
Do you not have friends etc who can support you? I think it's a lot to put on one person to make them your only emotional and practical support, and in a way, it's almost inevitable in this scenario that you might end up feeling like you aren't a priority for him.
Does he ever show up for you when you need him? If not, then don't marry him - he won't change, and you'll be setting yourself up for a lifetime of perpetual disappointment. If he does, then think about the times when you've felt let down and whether you might perhaps have been expecting a bit too much?