Unfortunately, we cannot possibly attempt to answer whether your Niece is having medication side effects on this forum. It is unclear if your niece had these issues to begin with and the medication is attempting to even her out, or if they are side effects of the medication.

I can however, share some similar symptoms that may help your understanding on what might be going on with your Niece.

My son, who has ADHD nibbled his nails down to nothing before he started medication and it hasn't changed now that he is on meds. I believe it's a nervous habit that began because of excess energy. I did the same thing when I was little, to the point my fingers would hurt and bleed but I couldn't stop. Potential sensory issues can affect food interest as well (smells wrong, tastes bad, texture yucky, etc.) which is another fun symptom of ADHD.

Nightmares are a somewhat normal, but no fun, part of childhood but your niece may have anxiety issues as well, which could be separate from her ADHD. It's called co morbidity when two (or more) disorders are found in the same person. (I have ADHD and Chronic Depression for example). I don't know if you knew that so forgive me for lecturing if you did. Her parents may or may not be aware of that potential and it might be something that needs to be discussed with her doctor if he is not already aware. In addition, bed wetting could be related to an immature bladder, which is something physiologic not related to ADHD, which you can also ask her parents about.

There is also no way of knowing through your post if 55lbs is natural for her body make-up and genetic disposition. Your side of the family could be normally built but her other genetic influences could be making her very slight. She could be an Olympic gymnast someday perhaps! She could be perfectly healthy for her age regardless of her ADHD status, I do not know. 

I do know that Ritalin and Concerta are basically the same medication, Concerta is a longer acting steady-release form of the Ritalin (same family anyway). Ritalin is a fast acting stimulant that also wears off rather quickly, 3-4 hours. I don't know what the treatment plan for her is but the Ritalin at night could be affecting her sleep, but I don't know her metabolism and she might be burning through it before bed. I metabolize my Adderall relatively quickly (a dose only lasts about 3 hours for me vice the 4-5 hours on most) a lot depends on her personal symptoms and metabolism. 

The best thing you can do for your niece is to open a dialog with her parents expressing your concerns and how much you care about her health, and asking if they would share her treatment plan with you. It's hard enough for parents to make the decision to add medication to what is probably already a stressful parent-child relationship, (ADHD can be very stressful on the relationship as the child CANNOT control their impulses and those impulses can be VERY disruptive, to put it lightly), without well-meaning outsiders (related or other) questioning those decisions.  BELIEVE me, it's torturous to see your child miserable with their inability to control themselves, discipline does not fix the problem (can make it worse), and be unable to help. It was a very difficult decision to give medication to our 4yo but with the negativity he was experiencing from us for his acting out, and from outsiders, we needed to help him. We do not want him to have a negative self-image.

So again, please dialog with your niece's parents in a non-confrontational way that lets them know you only have her best interests at heart and are concerned about her weight and emotional difficulties. Find out if this is what they are trying to treat or if perhaps the symptoms are side-effects they were unaware of.

I wish you and your niece well!

familynut