*@mrsm43s
But @ Norma27, why do you want the shit to hit the fan? Why do you want to cause trouble? That's so petty, and spiteful.
Why is it ok for SIL to exclude OP's youngest but petty and spiteful for OP to exclude SIL's youngest? confused*
To deliberately exclude a child in order to upset someone and make "the shit hit the fan" is spiteful.
To not include a 4 year old in age inappropriate activities, because that's just how it is due to her age, and to organise activities that suit the age of your child (or the children you've agreed to mind) - not spiteful.
OP seems to be even more petulant now - if she can't have her way, then " there will probably be very little relationship at all with any of them as we fill up our days already." Why? Why not just let your eldest go, and accept that your youngest will get her turn. Or the many other things suggested, such as arranging something specifically for the 8 year old with your 4 year old, separate to the 12 and 13 year olds, or inviting MIL out for the day, or accompanying your 4 year old, or DH taking your 4 year old round etc etc. You don't seem to be able to see it from the the other side or make any compromises at all. My way, or no way.
And with the Great grandchild coming along - surely that's your opportunity to build a younger family group that's more age appropriate for your youngest DD?
Your SIL and MIL are not being unreasonable. Your 4 year old is too young to be included, at this time, in the things they are arranging. But as she gets older, and more independent, the group dynamics will change, and you could be working to develop better links between her and the 8 year old. But that's for you to do, because its solely for the benefit of your child.
By all means deliberately screw up the relationship with that side of the family, its absolutely in your power to do so. I just don't understand why you want to.
You've already said that you think MIL would have your youngest if you asked - so if that's what you want, then ASK.