This is not a rare occurrence.
In some households, it is necessary for every working individual to co-sign on the loan to achieve the income level required for the bank to finance the loan. If the bank requires $4200/month gross income and the parent earns only $2000K/month, an adult sibling earning $1500/month might also sign. If a high school offspring earns $800/month from her part time job, she might also sign on to the loan so the family can become homeowners.
I know families where the recent HS graduate offspring sign for automobile loans for parents whose credit isn’t good enough because they struggled financially while raising the kids. Trucks to help fathers start landscaping business, minivans to help mothers with their cleaning businesses. In some families, the kids are appreciative of the struggles the parent(s) endured to raise them and are excited to be able to help out with the dire family finances which will help give younger siblings a better life.
Also, it could be that her income is needed only to be approved for the loan, not necessarily to pay for the loan. It’s often a complicated financial situation and not always easily understood by those who exist in higher SES conditions.