Posted: Sun Apr 5, 2020 7:15pm
Who has told you that you can't stay more than 30 days?
In normal circumstances, which these are not, after ninety days, or three months total, you would need to apply for residentia to allow you to stay legally in Spain. I think it is highly unlikely that appointments are currently available, before the crisis there was a delay in appointments.
Hopefully in thirty days time there will be light at the end of the tunnel, you will be able to change places with your sister, and all will be well.