We're organising this medium size party. DH and SIL don't have the closest relationship and in general they only show up because they drop off presents from my MIL but in 5 years of being together, I think she's only been with us socially at home once (not just because my MIL was here). So maybe it's no surprise that she's not coming, but she doesn't have an excuse, so I've reached a point of why bother?