It's obvious from your post how much this change is upsetting you Harriet; I am sorry.
In your position I would ask my ex d.i.l. if there is a reason why contact with your GC seems to be diminishing. It could well be as OCeanMama has said, that she's concerned, moving forward, that the relationship you have with your GC will not be the same as the one, if any, you'll have with your GC's sibling.
Whatever the reason, you deserve an explanation. As OceanMama has posted Ghosting or leaving someone hanging isn't kind and in your case I'd go further and say it's cruel.
Angela Rayner lashes out and calls Sunak “pint sized loser”.