Only one fraternal twin balding

I understand that fraternal twins have completely different genetics (unlike identical twins). However, from your experiences with patients, if one fraternal twin is suffering from MPB at a young age (20), will the other twin be more at risk to suffer from hair loss in the future?

That is not true. The genes for balding might have been inherited in only one of the twins.  The risks, depending upon the family genetics, can be very different for the second fraternal twin.  If they were identical twins, then they would both bald of not bald in the same way. I had a pair of twins, where one lost more hair faster than the other. Stress and hair systems was the cause for the twin that lost much more hair than the other twin.