lolais -- I must echo others here and I too am saddened by your apparent attitude. The idea that you view your grand-children differently based on DNA is gross at best. And as sodapop said, "I deplore your attitude to your grandchildren."
Do you view how your daughter treats her children as 'wrong' because she shows them 'too much love'/overcompensates for not being biologically related to her 'true'/real or your opinion? Is your bias and perception tainting your other children toward their sister and her children? Since it seems that the children of your children rank differently based on their DNA, is it possible that the other "loved grandchildren" who I assume were conceived 'normally' and not from a same-sex couple, is that possibly a covert reason for not embracing these grand-children?
You say, "We have just had a disastrous holiday because of the tension, and my daughter’s need to put her children above all else. I must admit I do find it very difficult to be even fond of them let alone love them." -- I can imagine there would be tension if your daughter and partner/wife were present with their kids, only to witness them being treated differently and without love or affection or even a desire from You to be "fond" of them when you admit love for your 'other' grand-children -- the biological ones.
As petcaf noted "Need to try and drop the non biological tag and just see them as your grandchildren."
trisher said, "I don't understand what biology has to do with anything. I don't understand why your daughter spoiling the children is responsible for the rest of your family not caring for them. It's not their fault."
As a personal note, as someone who has zero biological 'family' on either side for my children, my husband and I were adopted/embraced by Our OF (Other Family). Our OM (Other Mother) is a family woman. She embraces all the 'new people' to the family. If a child calls her Gran? THEY are her grand-children, Our clan included. She treats her grand-children (adopted and bio) the same. Grand nieces and nephews the same... as young children who deserve love and when given/shown 'it' thrive. Everyone should strive to be like Her.