@Foreignmumof2 I agree with you to a point.
If the two women don't particularly get along but can remain polite, then absolutely it is the best thing to have the children around each other.
But that isn't the case here.
Her SIL feels the need to be really rude and cause an atmosphere when they are together.
To such an extent that the OP no longer wants to continue to making an effort.
This is on the SIL and she clearly doesn't care if the cousins see each other.
Neither do the grandparents either.
If the grandparents did care they would ask that everyone remain polite and civil in their company.
But the grandparents care about themselves and what they want first.
I wouldn't send my children anywhere that I felt I couldn't go because someone felt the need to be so rude to me.
I wouldn't want my children to see me accept such treatment and for them to be around such an atmosphere.
If the grandparents offered to host the children without the parents that would be different.
Why is it on the OP to be put in self exile because her SIL is batshit.
The grandparents are responsible for this by allowing it to continue in their home.
Their priority is the other grandchildren.
Golden child dynamics.