Moving a fair distance away from your own friends and family is a very hard thing to do at the best of times. I know, I have done it a few times. It can be very lonely and isolating, and takes much, much longer than many people think to settle down and make friends/ networks and feel settled.
In your case this has been exacerbated by the fact that you were led to belieev that there would be some support and a network in place, and that has fallen through. You must feel very disappointed and given that it is your famil probably a bit bitter about it. Totally normal.
The stuff about your grandma won't be helping either. I'm sorry and I hope she is able to get better soon .
To be honest I think the fact that you have been left out of a big family celebration by your DH's family is pretty low. And I have 2 very different feelings about it.
THe first is that they are very used to you not being around, and will be used to organising and doing things without you. When I cam home from travelling even some of my best friends would do things like this. It takes a while for you to come back onto their radar and stay there. In this sense I have always thought that (and I realise this is a bit unfair) it's you who needs to make the effort and make sure they keep remembering you, even if it's only a phonecall or inviting them for a coffee. Have you been making the effort with them?
Secondly though, I think for a big family event like this you would have to try pretty hard to forget an intergal part of your family - so I do wonder if there is something more there.
However - what can you do about it? Someone on here recently metioned to 90/10 principle - google it. I think it applies here massively. It basically says that what you make of life is 10% about the things that happen to you and 90% about how you react to them. So how do you react to PILs apparent snub??
Personally I'd say ignore it. Will raising it with PILs help? I would think probably not, although it has a good chance of resulting in a family argument and rift. I wouldn't spend a great deal of effort thinking about and mulling over people who don't want to include you and your family - it's their loss. Instead, use this time to focus on settling into your new home. Find some groups to go to, join in with things at the DC's school if they are that age, maybe try meet ups from here etc. Try to focus on building yourself a new network.