Document to prove brother & sister relationship

Hi,

For my sister's standard visitor visa, I am bit unsure of document to show our brother - sister relationship. Although our passport backpage copies show we share same parents.

We did not have any birth certificates. The rules on gov.uk tells not to submit notorial certificates.

Therefore, I am thinking to submit our High school certificates showing our parents' name. Will that be enough?

Any help will be much appreciated

vikram

I submitted our birth Certificates. We also included family photos.