I'll first begin by saying that the language is confusing sometimes because you are talking about a fairly small (but I think growing) sub-culture, one which because it is small is more internationally sensitive (able to be affected by) when it comes to it's lingo and one which has not been blatantly established to be honest.

An Adult Little Girl is someone who feels like or believes themselves to be, or whatever word they use to describe it, a person who is mentally, in some ways often developmentally, a child, but someone with an adult body. And in this case, someone who feels female (at least when they also mentally feel that age). A Sissy (omg I hope I get this one right enough) is someone who feels "male", but who enjoys either being submissive or feminized in some way, most often this is confined to the bedroom or the house, or some confined space. Few people are full time sissys and few people seem to be full time adult children.

An Adult Baby is similar to an Adult Child, but one who regresses much further back in development than most adult Children. Not to say people who are Adult Children do not regress sometimes back to this state of mind, but the defining line lies in the person's interests.

People who are either full time or who regress to a state of babyhood (0-3ish or so in age) would be called Adult Babys. People who like me who in my case pretty much know I am 12 and I am usually about that age, most of the day if not all of it are Adult Childen.

Adult Children can wear diapers and not be Adult Babys.

Adult Children, Adult Babys and Sissys though can be of any gender and or combination of orientation. Traditionally a Sissy has been a Gay man, but you can be a Sissy and be Bi or Straight...or somewhere else entirely. Some Adult Children and Adult Babys are sexually active, some are not. Some are sexually aware, some are not (most are though).

A Female Sissy would be usually refered to as a Sweet Lolita in terms of dress and they certainally exist. Most people know what Gothic Lolita is now (I think), Sweet Lolita is the Strawberry Shortcake version of Gothic Lolita and the closest thing I can think of being a Female Sissy. I don't know that there is a proper term for a Female Sissy, but you can sure have fun inventing your own term if there isn't 

One last thing to talk about is that people have varying levels of involvment with their young or their "adult" self (we can't forget the "Mommy" and Daddy"s out there). For some people it is something they do on the weekend, for others for part of the day and for some they live like that all the time. But usually your life has to be going really good or really bad for you to go full time. Going full time though is not usually the goals of most of the people, for me it was a second moment of self honesty, one which I could not ignore a second time.

I hope that helps clear things up for ya 

I fully hope that other people will throw in their two cents to the definitions. I come from one perspective, from one place and am only one of the types of people I was talking about. The more words are used though, the more they will cement themselves into the global lexicon so that one day it will be less confusing for people.