There is a lot of back story here but doesn't actually affect or justify these events and would be too long to write down.
It is about my in-laws and I find the 'SIL, IL' etc. acronyms confusing so I am going to write the issue from DH's perspective, he wants everyone's opinion.
DB lives in a different country in the UK to us for work. DM/DDad, DS/BIL and DW and I all live equidistant in a triangle from each other. Last year on Christmas eve we saw an instagram post of DB, DM/DD at DS' with DN who was a baby, at a restaurant near DS' house celebrating Xmas (all spending the actual day separately that year). I kicked off as I thought it was harsh to find out via instagram that I hadn't been invited to a Christmas event and it felt very personal. DM had mentioned going to DS' but had said it was purely to see her and the baby before Xmas (we were seeing DM/DD boxing day) she also said DB wasn't going to go and they had no idea what was happening. I left it after the argument but DM blamed DW (MEEEEE!!) for putting the idea in my head that my family treats me differently, which isn't true. This is the background I won't go into but it has always been said by aunts/uncles that I was treated differently/unwanted but we all had a good upbringing so it just gets dismissed.
DM accidentally has forwarded me an email that she is unaware of doing so (DS and I have the same email except the initial is different and next to each other on a keyboard). It is from DS sending DM a timetable of all the times DM is looking after DN and when DS is going to DM's. For the time leading up to Christmas it says 'DS hosting pre-Xmas with mum, dad and DB'.
This means that the exact same event is happening this year that we haven't been invited to. If we confront now, they'd invite us and it'd feel fake. So do we bide our time, see how it plays out and if the same occurs we finally have proof?
WWYD?