There is no special Rollup column type in coda.
Any formula can refer to ANY property of ANY relation using the dot-notation like this;
Product.Price
So in your example the formula for Grandparents is…
Parents.Parents
And use the same notation for all the other relationships.
So the formula for Siblings is simply…
Parents.Children
However, these formulas will return lists-of-lists, so for complete correctness, we should add the formula .ListCombine() to the end, so that it will collapse the list-of-lists into a single simple list.
So the final formulas are
Grandparents; Parents.Parents.ListCombine()
Siblings; Parents.Children.ListCombine()