As with any Narcotic pain killer, the risk of addiction is always quite high, & if your family member has chronic, long-term pain, developing a physical addiction to Demerol or anything else that will adequately control his/her pain is pretty much unavoidable. As far as addiction in terms of "recreational benefits" & all that are concerned, the risk of developing an addiction to Demerol is definitely less than the risk of developing an addition to something like Oxycontin; it's more along the lines of the risk of developing an addiction to Vicodin or Percocet [Percocet in 5mg-10mg dosages, that is].
50mg of Demerol is actually the lowest dose of Demerol when taken orally, I believe; oral Demerol dosages generally range between 50mg - 150mg. Since Demerol is incredibly short acting (it usually lasts for about 3-4 hours), taking four or five dosages of 50mg of Demerol a day is completely normal for someone in extreme pain.
Your concern is, of course, very natural. It sounds like, though, your family member is controlling his/her pain in the best, safest way possible.