Hello ClaraFavata
Thank you for your message. Unfortunately, the Youth Tandem Day Pass does not offer a way to bring a companion if the GA holder is over 25 years old. Since your sister is under 26 but you do not meet the age requirement, the Tandem Day Pass cannot be utilized in your situation.
I understand that this is not ideal. While the provided information does not include alternative solutions for discounted tickets, I recommend regularly checking the SBB website for any potential offers or promotions.
Please feel free to contact us if you have further questions or need assistance.
Best regards,
KadijaR