Unfortunately this is life, sad though it is, and perhaps a bit more extreme. My son has been sleeping on the sofa for about five years although they go to some kengths to hide the fact when I stay. At one time early on he told me it was because he can't get to sleep without the tv on, but DIL will not admit it even happens. A couple of years ago they announced that they had decided not to have a second child - my grandson is 12 - which hardly surprised me in the circumstances! I get on OK with DIL, she makes an effort to me nice to me but it wears thin if I stay more than a couple of nights (they live too far away for a day visit), or if they come to me.
Regarding the grandchildren, I grew up being called daddy's girl because my younger sister was mum's baby, although she was not spoilt in the way that OP's GD is. But much as I loved my dad he was quite a remote presence and I longed to be closer to my mother, which only happened after we grew up and left home. At least the boy has a gran to spoil him, bless him.
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.Their son was never allowed out of his room during the night after 2 years of age but their daughter has them wrapped round her little finger.
me (easy to do these days) ..Is she or isn't she well behaved at your house?