I had this exact same issue, with two month old goslings and then five hatchlings. I thought their natural parenting instinct would cause them to welcome the new babies but instead they attacked them and were complete snots.
These same geese accepted goslings from the hatchery when they were about a year old, parented them and protected them like the Pope. It was an age thing.
Perhaps keep them separated but visible to each other for a while, and eventually they'll all go together.
I just introduced 2 two-month-old goslings to our last remaining goose, a fullgrown male white Chinese goose, and surprisingly he took right to them. He protects them and takes care of them like a mother.
He's about one or two years old though, so I believe that it is an age thing and perhaps younger geese don't take so easily to others.
I introduce new outside goslings to the flock by turning them out in 10' circle of wire fence within the grazing range, while they are still very young 2nd week if the weather will allow it. Sometimes there is a hostile reaction at first but most times the flock just wants to adopt them. I do this daily until the brood period is complete. 4 times out of 4 they were able to join the flock with no problems.
When natural born goslings arrive the parents introduce them to the flock and their standing in the flock depends on where the parents stand in the pecking order. Low status goslings are sometimes pushed out of the way of high status adults but I have never lost a gosling to goose attack. they are very much flock animals.