Since your brother is inviting, it would be his invitation letter inviting parents for graduation. I'd also suggest he get a letter from university confirming the convocation ceremony. Although he should also mention about you and give details about you coming for his convocation and meeting your family as well (shows stronger ties/reasons). For funds I don't have much information. If your parents can fund themselves then it's perfect. If you are funding then it gets a bit complicated for which if leave it to more senior/knowledgeable people on this site.