Hello IRF540,

I don't think there will be any problem since they have a multiple visas. But its smart to carry your invitation letter of your graduation even thought its already passed. Most of the time, the immigration officers might ask the purpose of visit. They can say its the family visit, as they have already the genuine visa.

Howover, since they have multiple visa entry, and I hope they been here already... it won't be problem anymore. Good Luck.