01. One of the Storms junior was adopted.
02. One of the Storms senior arrived home one one night with the resultant of an affair, and said to their spouse "Deal with it or suck it."
03. One of the Storms senior could have had fertility issues or they could have been a gay couple of Storms and they used a sperm donor.
04. One of the Storms Senior could have begun this new marriage with a child already tallied from a previous marriage.
05. One of the Storms Junior could have been Switched at birth and no one has noticed yet.
06. A Time travelling villain of the Fantastic Four genetically modified foetus Sue Storm until she was a weapon designed to destroy Reed Richards.
07. Johnny fell into a cauldron of bleach as a baby.
08. The Storms Seniors were scientists creating Super soldiers and decided to raise their peak human experiments as their own children.
09.The Storms Senior went to a lot of Swingers key parties in the early 80s.
10. An alien child cuckoo, was telepathically integrated into a typical human family to be raised sincerely as their own, because it's a cuckoo and that's their bag baby.
I really doubt that I could go on.