Oakland tenant not on lease or signed estoppel - can I kick out?

[Eric Chang]

I purchased a rental income duplex in Oakland September 2017. In one of the units, the final tenant on the written lease is moving out. However, there is one other person, Sara, who has been living in the unit since March 2017 and paying rent, however she is not on the lease. When we purchased the property we requested an estoppel. The estoppel form was filled out by the tenant and Sara, however neither the previous owner or myself signed the form, which makes me think its not legally binding. 

I'd like to remodel the unit, start with a new set of tenants, and raise the rent given that Sara's plan is to find two new roommates which would just delay my plans. My property manager says Sara may attempt to stay and it could become a bigger issue. They could try to make the case that they are tenants one way or another and given this is under Oakland measure EE, this could turn into a pricey time consuming process and result in an eviction.

I'm looking for advice on if anyone else has dealt with a similar experience. Please help!