One of the major things about the “Spencer has a twin” theory I can’t wrap my brain around is that the Hastings’ wanted Mary’s baby. They were 100% on board with taking Spencer and raising her as their own. No questions asked.
If there were another baby, wouldn’t they have taken her, too? The Hastings were well off and had plenty of money to raise twins alongside Melissa. Also, wouldn’t Veronica have mentioned another baby when telling Spencer about her birth mom?
It seems really unlikely to me that Veronica and Peter would just take Spencer and leave the other one to child protective services or whoever. Because surely it’s gotta be against policy for a Radley patient to be allowed to raise a baby in an insane asylum??