Okay, let's do some investigation. He's 29, she's 19, that's already an FBI-level age gap, and now the main thing, so it's his older brother, not younger, and her younger sister, not older. In conclusion
With parental consent, yes. The sisters' parents have passed away, so they cannot consent to marriage. Hence the younger sister has to be at least 18. They could be twins, with the FMC born five minutes before her sister.
Age of consent is 16 (used to be 13) and 18 is age of majority.