3 adults living in house, only 2 willing to sign lease renewal

[Marci Stein]

Lovely couple and her sister living in House . great tenants .
sister says she might move back to Florida one day and refuses to sign the lease renwal along with the others. 
not sure how to proceed.
I don’t see any negative impact of leaving her off the lease, but wondering if there’s something I don’t know.? she did sign it for the first year. Ttia!