What age does the city lux stroller go up to, please?
Depending on the size of the child or children, realistically, you can use it up to 2-2.5 years of age. You could fit a larger, older child, too, though he or she will be very tight in the seat unless a tiny child.
For an older sibling that is larger, I personally would recommend the bench seat accessory (only one can go on this, though).
Your -very berry- Eli.