It's sort of hard to say. With LSD at normal doses, there's always a chance of psychological issues with people who are predisposed, i.e., LSD could catalyze underlying mental instabilities. There is also a slight chance of physiological problems such as HPPD even in mentally balanced people.
Less is known about DOB than acid simply because the history of its use in the drug culture is far less extensive. Unlike LSD, it has caused deaths and physiological injury at doses beginning around 20 to 30 times the effective dose. In other words, if we take an effective dose to be 2 mg (and some people seem to be very affected at 1 mg), a dosage of 50 mg could quite possibly cause serious injury or death, and in some people this dosage could be even lower.
Assuming what your sister and her friend took was DOB, if one were to estimate that--on the outside--each blotter contained 5 mg (which is possible with the big rainbow blotters), then they would have each consumed as much as 12.5 mg of the drug.
It appears that such a dose for most people is quite a big one, and very capable of inducing vasoconstriction problems. But seeing as they both survived, in all likelihood any lasting problems will be related to the psychological trauma they experienced from such a dose.
So in my own inexpert opinion, the thing I would watch out for in the short- to mid-term would be PTSD.