Hello, I hope you are doing well

Yes, it is completely legal you can get a first graduate certificate if your sister hasn't received it. There are 2 conditions to get a first graduate certificate:

No one other than the person claiming to be the first graduate should be graduated in the family. If there is someone who is a graduate but hasn't received the certificate you can get it.

Since your sister is not having a certificate, you can get it.

Hope this helps.

Hope this is helpful!

All the best!