I guess to be able to chat about the uni with their parent/s after, and share opinions. Not entirely sure why a younger sibling, unless there was no alternative for childcare.
Some 16/17/18 year olds are shy, some may not have thought of all the relevant questions, some may just like to have an open discussion between lecturers, parent/s and themselves.
Our DS is in his 3rd year at uni. We, or I, went to all of the open days that he wanted to go to. His choice, but I was pleased to be asked. He did most of the talking though. Then we chatted about it all lots after the open evenings.
Why do you ask?