Is Alex Wesker Related to Albert Wesker? Not by Blood

No, Alex Wesker and Albert Wesker aren't blood relatives. Resident Evil Revelations 2 contains a memo stating plainly that the two share no blood, and it describes them as bound by being the last Wesker children alive. (Resident Evil Revelations 2, "Stuart's Memo: The Wesker Project")

What exactly is the link between them?#

The Wesker Project, also called Project W, was run by Spencer to produce a superior human race using the Progenitor virus. The wiki says children from around the globe, hundreds of them, were taken in, some legally and some not, chosen for what their parents had contributed to mankind. All were given the surname Wesker and educated at great expense while the project watched them. (Resident Evil Wiki, "Wesker Children")

By 1998 the project narrowed the field to thirteen of the best candidates and infected them. The wiki says eleven died, and Albert and Alex lived. Albert gained superhuman abilities. Alex gained none, because she had a terminal degenerative disease when she was infected. (Resident Evil Wiki, "Wesker Children")

So the link is shared upbringing and shared survival. Revelations 2's memo, written by Alex's aide Stuart, puts it in one sentence: she isn't related to Albert by blood, but the two seemed bonded by being the last Wesker children alive. (Resident Evil Revelations 2)

Was Alex like a sister to Albert?#

Practically, yes. The wiki says Alex was close to Albert and that the two were regarded as the strongest candidates the project produced. After his death in 2009 it says she was left saddened and lonely, having lost a person she thought of as a sibling and an equal. The Japanese text of the memo goes a step further. In Project Umbrella's English translation it says the two had no blood connection but that Albert was like an older brother to her. (Resident Evil Wiki, "Alex Wesker" and "Albert Wesker"; Resident Evil Revelations 2; Project Umbrella, "Stuart's Notes / Wesker Project")

They also made opposite choices. Albert built the Uroboros project to wipe out everyone unsuited to the virus. Alex started t-Phobos to escape her failing body by moving her mind into another one, and she'd later chase that goal through Natalia Korda. (Resident Evil Wiki, "Wesker Children")

Is Wesker Spencer's son?#

Not by blood, as far as any source goes. The wiki says Albert was removed from his parents as a young child by Spencer's agents and raised in one of perhaps several homes paid for by the Spencer Foundation. It also says he inherited advantageous genes from his parents. His early life is a blank, down to his real name. Oswell Spencer was the project's architect and patron, which is a different role from father. (Resident Evil Wiki, "Albert Wesker")

The memo's Japanese text does call Spencer Albert's "father and mentor" in Project Umbrella's translation, where the English release only says Albert worked under Spencer's tutelage. I read that as a figure of speech about his upbringing. Albert's only listed family member is his own illegitimate son, Jake Muller. (Resident Evil Revelations 2; Project Umbrella, "Stuart's Notes / Wesker Project"; Resident Evil Wiki, "Albert Wesker")

Albert learned the truth late. The wiki says Spencer, near death, arranged for Albert to visit his estate, where he was told about the Wesker Project. Albert hadn't known the B.O.W. research existed to feed a eugenics project. He killed Spencer. (Resident Evil Wiki, "Albert Wesker"; Resident Evil 5)

Who is Alex Wesker?#

An Umbrella Pharmaceuticals scientist and a senior member of its Intelligence Division, who by the 1990s knew about Project W and knew Spencer personally. She later ruled Sein Island, where the plot of Resident Evil Revelations 2 takes place. The wiki gives her death as August 2011, aged 51. Her full story is on the Alex Wesker page. (Resident Evil Wiki, "Alex Wesker")

My read#

People keep calling Alex Albert's sister, and I understand why, since the wiki itself says she thought of him as one. But the sources I checked stop short of making it true. They were two children, selected by the same man, who ended up with the same name and mirror-image ambitions. That's a stranger link than blood. (Resident Evil Revelations 2; Resident Evil Wiki)