The type of epilepsy DD#1 has been found to be correlated with a number of things, including "speech sound disorder". In fact, though the research is in its infancy, there have been a number of studies publishing showing a high - sometimes equally high - correlation between being a sibling of a kid with the epilepsy and particular problems. In other words, a sibling without overt seizures is just as likely to have problem X as the epileptic kid herself. (DD#2 doesn't have seizures that we know of, and she's never been tested for the characteristic EEG pattern that goes along with this epilepsy.) So there could be other problems that are more common with siblings that just haven't been researched or documented yet (and DD#1 could also have problems that were never diagnosed, of course).