Why are you joining as a "late co-founder"? Was the team that was lost included the first co-founder? How well do you know the CEO? Have you worked with him/her

Why are you joining as a "late co-founder"? Was the team that was lost included the first co-founder? How well do you know the CEO? Have you worked with him/her before? There are so many red flags that I'd flee the sinking ship and go with the other start-up.