DH and I were talking about DD's university graduation next year (assuming she passes her final year, of course, fingers crossed). We're lucky that her university allows each student to have several tickets so DD can also invite extended family. Absolutely no expectation for anyone should attend, but it's nice that DD can ask them. Her university isn't close to anyone's home, so we'll all have to travel, book accommodation, pay for meals, etc.
One of DH's sisters loves family events and we're pretty sure that she and her DH will want to come. The only slight drawback is that they also have a history of freeloading when travelling with family members. No logical reason for this, they have professional jobs and go on several holidays a year. I think they just don't like spending money!
We'll pay for a celebratory family meal, but how do we gently get the message across that if they do decide to come, they'll have to pay for their accommodation, meals, etc. DH's brother got really annoyed with them on a recent joint holiday!