At this time we don't have any advanced setting that will recognize siblings and combine their invoices. The best thing to do would be to create a separate invoice for each child - when the parent goes to pay, they will be able to click each payment and hit "pay now." If the parent has set up Auto Pay, then it will be seamless on their end. With this method, your records will be up-to-date and accurate for each child's tuition.
If you do want to combine them, you could add the second sibling's name as a line item on the first child's invoice. This can get a little tricky or confusing for parents, but has worked for some schools.