some of our breeder trainers encourage it , outside the presence of the mother ,one at a time. This will familiarize the pup to the dog and will make it easier when you bring one of them home because they have already met . In most cases where problems arise ,it's the younger pup that initiate it.
The big problem (besides how the mother and the adult dog feel about the whole thing) is that it is right at the pup's most vulnerable time for infection. Few breeders want to risk their entire litter allowing a strange dog into the house.
Fortunately, we have a breed that, in general, is pretty social. I really haven't heard of a well bred Havanese puppy, introduced into a household with other Havanese where there was a problem at the puppy stage. I haven't heard of anyone having a problem in a pet only home at all with dogs that come from good breeders and have been properly socialized. I've only heard of a serious problem with adult Havanese in the same household three times. Two of those times have been with intact dogs.
In one case an adult father and son, in the other case, two bitches. The father and sone were intermittent, and mostly a lot of yelling and carrying on. The bitches are totally serious. They have to be kept in opposite ends of the house. They try to kill each other. I've suggested that they re-home one of them. Unfortunately, one is the wife's favorite, the other is the husband's favorite. Makes for an interesting family dynamic.
The third case was one we know about on the forum, where one of the dogs was a rescue, (maybe both... I don't remember about the older one) and had shown herself to be pretty unbalanced from the very beginning. Problems just got worse as she got older.