In the 1911 census my grtgrandparents had an "adopted son" aged about 14 in it.
His surname was not the same as theirs but, after a lot of searching, he looks as if he was the son of married cousins who both died.
My guess is that he was informally adopted, partly because he was the same age as a son my grtgrandparents had lost to typhoid.
Also, I have a friend age 65 who is sure that he is the son of his father's unmarried sister. Noone ever said anything to confirm this but he looks very like her, she was very close to him, financed his education,left him all her money, and his only "sibling" was 15 years older than him.Iwould have been relatively easy, he thinks, perhaps with the connivance of a private doctor, to pass him off as the baby of his real mother's brother and sister-in-law.