Normal 3 year old behavior?

[KaylaMarie25]

My fiance has an almost 3 year old from a previous relationship. He'll be 3 in October. He's generally a typical 2 1/2 year old, with tantrums and in the whole "mine phase" or saying no to everything. But he patience isn't his best virtue. When he wants more juice or water he'll literally throw a screaming/crying/hitting tantrum if you don't get up right then and there. We're trying to teach him that throwing tantrums doesn't get what he wants and he needs to say please. sometimes he'll say please if you tell him too. My fiance's aunt thinks he may be bi-polar, though it's WAY too soon for a diagnosis. One minute he's laughing and being goofy then out of nowhere he'll just start screaming and crying for no apparent reason. He also lives in a house where there is a 9 year old girl who has an "attitude problem" and disrespects her elders (me included). She's my fiance's cousin. If she doesn't want to do something you ask her to she'll either ignore you or say no. I personally think that Brandyn is picking up on this behavior and that's why he does this. She's also very mean to him. And I've caught her yelling at him and hitting him. Then she gets even more mad when he hits her. I've tried telling her that if he's acting up to tell an adult and let one of us deal with it. When I'm there, there is a total of 3-5 adults there. My fiance's grandfather is 100% blind so disciplining Brandyn is a little difficult for him. Brandyn generally listens to me an my fiance's aunt more than anyone. He's very smart for his age. He can count to 10, knows his animals and animal sounds, his alphabet and he can count to 5 in spanish. From what I've heard Brandyn wasn't a preemie. But he was 6 pounds (some odd ounces) and 23 inches long and on monitors for the first 2 days. Also, sometimes he REFUSES to take a nap and then stay up till about 11pm-midnight. The house calms down around 8pm. He'll wake up every couple hours and stay up. We don't think he's having nightmares, he doesn't thrash about in his sleep or make whimpering noises as most people do when having a nightmare. This happens even if he does have a nap sometimes. My fiance and I are trying to save up money to get our own place. We both think that this behavior is due to my fiance's cousin. While his aunt thinks he may have a disorder of some sort.  Any input?

PS: I've posted about my fiance's son before with him not talking. Now, he will talk your ear off lol. He says: dad, mom, ma, mom-mom, beth (my fiance's cousin), dude, hi, hey, duck, cat, couch, cow, tree, please, thanks, dog, spongebob, cup, eat, bath, food, milk, juice, candy, mio (that water stuff), rose, outside, car. And more. He can also form sentences such as: I want that, Cause I can (he's a smart-butt), I'm bored. And more lol.

[KaylaMarie25]

When he sleeps he doesn't make odd noises, snore or have trouble breathing. My fiance thinks it's because everyone is still up and Brandyn doesn't want to feel "left out". Also, the 9 year old, is very rude and has a horrible attitude. Last time I was over there I tried telling her something and she said " I don't really care, I hate learning when I'm not in school" and then throws something at me. And if you ask her to do something that she doesn't want to she'll say you're dead to her. She also tried to choke herself and then told her mom (my fiances aunt) that my fiance's 16 year old brother did it. I think she could get away with murder. My fiance and his aunt have two different styles of parenting. Since Beth (the 9 year old) is his aunts "baby" she gets treated like one... a spoiled one. She should know better not to do/say half the stuff she does. Brandyn on the other hand isn't even 3 yet, so it's understandable for him to push his limits to see what's right/wrong. We just don't want Brandyn picking up on this. We're trying our hardest to save move so we can get our own place.

[Sandman2]

   Very smart kids learn quickly what works for them to get there way.  Also kids at an early age communicate by crying, screaming etc. cause they can't talk very well.  If that works for them - they keep doing it.  
   I do worry about the not sleeping at night.   Lack of sleep will dramatically effect a child's behavior during the day!   Does he snore at night? or seem to have trouble breathing.  Its probably worth having his tonsils and adenoids checked out.
   Brandyn is not picking up on the 9 year olds behavior.  He is picking up on the discipline procedures that allowed her to get the way she is.   I would recommend getting the book, "SOS Help for Parents," by Lynn Clark.  Its got a very good system that will help with both kids.