Invite or not the parents to graduation?

Hi! I've got a hard dilemma. I have been thinking about it for weeks but I can't find a right solution, what to do. If someone gets any idea about that problem or has been in similar situation, please write to me. The problems are: my mom has a mental disorder and I daren't invite her to my graduation because nobody knows what is the impact of this event for her and how she reacts. She is righteous and sometimes she communicates with the god... My father is 20 years older than my mother and has some health problems but he also wants to come congratulate me. I haven't told my dad a clear desicion about that yet but I have to do it soon... I think rather my father could "survive" this event than my mother but I can't on no condition invite only one of them - no matter which. Me and my brother discussed it would be better for everyone if only my brother comes to graduation and after that we go home for a little dinner party. But... both parents had helped me a lot materially and emotionally during my studies and private life's hard times at the same time. It seems this occasion means a lot for them. It's like... our collective victory. H o w could I do that for them? Should I still invite them? What is the possible price for me to pay if something goes not the way it should? Should they understand my position? Give me any advice and share your thougts please.