My sister and myself went to the concert.
My sister and I went to the concert.
Myself is a reflexive pronoun. It cannot be used instead of the subject pronoun I.
Use the subject pronoun I, not myself, when the person is the subject of the verb: ‘My sister and I went to the concert.’
To check, remove the other person: we say ‘I went to the concert’, not ‘myself went to the concert’.