How rare is littermate syndrome?

I had to get the definition of Littermate Syndrome and thought others may want to see this link about it.
Link:
https://k9behavioralservices.com/littermate-syndrome/#:~:text=Littermate%20Syndrome%20(also%20knows%20as,age%2C%20when%20puppies%20are%20usually

According to that article it states NOT to have sibling puppies in same household, but since your father won’t separte them then HE take should take the FULL responsibility for the result of having them together.

Perhaps move away from this problem which will occurr soon enough so that YOU alone will not should the unwanted bad behviours, which one day may harm a visitor or worse a relative.
( court action may result is someone injured because of his decison to keep them).

It is quite common and not recommended to have puppies near the same age even if they are not siblings.
When two puppies are placed together, they learn to rely on each other. One of the puppies always becomes shy, even when both puppies started off as bold and outgoing. This is a HUGE problem, since it means that the shy puppy never reaches his or her potential. In fact, this was such a major issue that the guide dog experiment was quickly halted, and to this day Guide Dog Organizations only place one puppy at a time in puppy raisers’ homes, even when the homes are highly experienced.

Other heart-breaking effects of Littermate Syndrome:
The shy puppy becomes increasingly withdrawn and introverted and never reaches potential
Often even the “bold” puppy turns out to be quite nervous and uncertain when separated
The puppies often become incredibly co-dependent, exhibiting high anxiety when separated
They often fail to bond to their human family as strongly as they otherwise would, if at all.
At social maturity, these puppies may begin fighting with one another, often quite severely.
Even non-sibling puppies can exhibit Littermate Syndrome when placed together.

Professional trainers recommend against getting two puppies within six months of one another, because the risks are just too high. This doesn’t even take into account the other practical considerations, such as the increased costs of vet care, food, supplies, and training; the extra work of training and caring for two dogs; or the time requirements of two active puppies.
Soure

Common. What if you tried to find people (friends, relatives, neighbours) to take an interest in just one of the puppies? Give each puppy a sort of “godparent”? Maybe your dad will accept that coincidentally, friend A loves puppy number one, while the neighbour somehow only wants to walk with puppy number two…? You can certainly find dog-friendly people while these pups are young. Just a thought.