Do not expect your g.son to be happy about having not one, but two babies in the house taking so many peoples attention and time. And, twins do attract a lot of attention.
Do expect some retro behavior as he may want to become 'a baby' again.
it will pass. For yourself and the parents to be I would suggest that you join TAMBA, and be able to get lots of help and advice from others who have had multiple births.
They produce an excellent magazine and most areas have their own branches which used to meet regularly for self-support. Not sure what happens with those at present.
Multiple births are so exciting - hope you daughter in law can really enjoy them.
When my identical twin girls were born, I had three others, a 5 year old just started school, a four year old and a two year old.
I became an 'earth mother' and when I fed the babies from the breast during the day I would do it mainly on my bed so that the 2-year old could sit in front of me and I could read her books to her (upside down and mainly knew by heart).
Had a lovely friend who would come over each morning to help me bath the babies allowing them both to have a feed from me by themselves.
Breast feeding may be difficult to get establised, but is so worthwhile and saves so very much time.