Aggressive Niece

[lvofjc]

Hi, my boyfriend's sister, "Melody", has a daughter, " Kelsey", who has adhd.  She has taken meds for it for the last year, (she will be 7 soon). Anyway, I was talking to one of our friend's, who has a daughter, "Ally", who plays with Kelsey and she said that Ally didn't like playing with her, because she hits. And when our friend said something to Kelsey's mom, she ignored it, and didn't get on to Kelsey for it.
Then the other night, when Melody and Kelsey was over at our house, Kelsey got a hold of a plastic bag and was trying to put it over her head, she was calling it her hat, but her mom got on to her, but Kelsey wouldn't take it off.  Finally, my boyfriend, her uncle, got on to her, and told her to give it to him now!   Kelsey got very mad, and tried to rip the bag, without much effort, and ended up throwing it away, then she stormed into my son's bedroom.  My son went after her, and wanted to play.  Melody kept turning her head towards them, and finally said, she has to watch Kelsey, because when adults get on to her, she will sometimes take it out on the children around her.
This upset me, because already in the past several months, I have notice when Kelsey gets mad at her mom, she hits, or bits her, which her mom tells her to stop, but that is it, and Kelsey doen't stop until, the uncle or I say something to her.   Plus, sometimes Kelsey makes some of the meanest, maddest faces I have ever seen in a child.  
My question is; is her meds doing this?   I don't think Kelsey is a bad person, but is her meds doing this, or is it her mom lets her get away with it, because she thinks she can't say anything to her child because of Adhd?   Or in other words, is it a lack of disciple?
I believe every child should have a set of rules, whether they have Adhd or not.  
Should I let them come over, because the second Kelsey bits or hits my son, I know I will get mad at Melody?
I know kids fight sometimes, but if Kelsey fights with my son, because she is mad at her mom or uncle I won't be able to let them come over.  Kelsey is usually very sweet most of the time, and my son and her get along about 90% of the time.
So, should I say something to Melody and how should I say it?
Surely, just because a child has Adhd, doesn't mean they can get away with aggressive behavior.  From what I have read, it sounds like they need more of ground rules.  Nothing harsh, just that no means no.  Of course, most kids have to be reminded of that sometimes.
Any answers would be great.  
I just need help in finding out how to deal with it, if it becomes worst!
Thanks.

[lvofjc]

Thanks for your comment.
I notice that it seems to be at just different times.  I normally hate giving a child meds, but do you think her dose is not strong enough?
At Christmas, it was before we ate (noon), we let all the kids open gifts.  She also hit her mother that morning, and when I looked back through the pictures, I noticed Kelsey was making very mean faces. I thought maybe she just didn't feel good, but over the last month, I was very surprised at her attitude towards her mom, and I was taken aback by the fact that Kelsey was biting at her age.
Her parents are not together, but Melody said they get along better now then they did when they was together.  
I really do enjoy having them over, but Kelsey seems to have mood swings throughout the day.  
I know kids get mad, and argue, and fight sometimes, but hers seems to be aimed a lot at her mom.  That just really surprise me!  
It also concerns me, because I am having a huge birthday party in the spring with a lot of children.  I really would like Kelsey to come, but I also am not sure.  
I know I can't predict what will happen, I guess I am just looking for reassurance.  
Do you think I should say something to Melody about her daughter's attitude towards her, maybe Kelsey needs a time out everytime she hits or bits her mom?  Or should I say to Kelsey that it isn't very nice of her to do that to her mom, and she is hurting her mom by hitting or biting her?
I know being a single parent is hard, I've done it.  
I know Melody needs some support from her friends, I just don't know how to go about it, without upsetting Melody.  
Or I'm I just over reacting, or is this more about Kelsey's attitude towards her mom?
But how do I reassure my fears, and help Melody if she needs it?