K.,
Don't fret. Chloe is nearly 2, she is in completely new surroundings that are different from what she has known all her life. I am of the opinion that children are very resilient. You are wise to realize that she has not been around a lot of children, other than her Gym friends, and you are always in that mix. Apparently you trust this new sitter, or your kids would not be there. Trust her to let Chloe grow, and adjust to the new environment at her own pace. Develope a 'ritual' for delivering her to the sitter. A special kiss, a special dance, or something to make the arrival fun! Then stick with it everyday and when you leave--just be very matter of fact, and be gone. She will cry when you leave, that's OK. But if the sitter spends her time trying to pacify the cry...she is really prolonging it. Just have her reassure Chloe that Mom/Dad will be back, and move on. It may take a while, but Chloe will learn that its OK, Mom/Dad always come back. Her playing alone shouldn't be an issue, be glad she can entertain herself... some kids must have a playmate or a parent in attendance all the time. This can get very tiring. As for the crying when the sitter puts her down, I believe she does it to get the sitter to pick her up again. And it works! By doing that, the sitter is teaching her that everytime she wants something all she has to do is cry, and she'll get her way. Kids are SMART! They know what works, and they use it. Chloe is also commanding all the sitters attention when she's being held, and it seems that this is what she wants. Don't try to label her. She sounds like a normal 2 year old with simple adjustment needs. All kids go through them. Eventually, she will see that the other kids are having fun playing together, and that she is 'left out' (by her own choice). When she realizes this, she will inclued herself in joint play. I believe that if the sitter simply notices her need for attention with a hug and moves on, Chloe will learn that the crying is not a proper tool to use for attention.(this will not happen in one day, or overnight. The Sitter must be on board to stick with it for a while, until Chloe understands) She needs to be noticed and praised/held/hugged when she is getting along and behaving well. Perhaps small rewards-out of the blue- for playing nicely or being happy instead of crying.(a happy face sticker, to match her 'happy face')She will learn to use positive behavior as a way to get the attention.
Remember too, that she is probably still adjusting to a 7 month old sibling...when you're 2, you still need a lot of attention, you're the center of the Universe! and baby brother has infringed on that a bit. Not to worry, kids have been adjusting to new siblings for hundreds of years...Chloe will be fine, it just may take time and positive reinforcement from you, your husband and the new sitter. I will pray that all the wrinkles are smoothed out soon. Develope a plan and stick with it! As adults, we sometimes decide after a day or two, 'This is not working'.... We must be prepared to out-last the child's button pushing techniques. Otherwise, the child is in control.