*Eres* most likely<sup>1</sup> comes from the 2nd person singular future active indicative of sum, *eris* <sub>("you will be")</sub>.

*Sois* is comes from the Late Latin form \**sutis* (formed by analogy to *sumus*)<sup>2</sup>:

> ESTIS (re-placed by *SUTIS, analogical to SUMUS) > OSp. *sodes* > ModSp. *sois*

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<sup>1. [*On the Origin of Spanish eres*](https://www.jstor.org/stable/475003), Griffin (1994)</sup>  
<sup>2. [*The Origin of Spanish "ser": A Phonosyntactic Analysis*](https://search.proquest.com/openview/2c6c45288813fd5bd7e9e1098c878fa3/1), Joel Rini (1997)</sup>