Hmm... I like what Phoniex said, but I want to out in my own two cents.
I am reading a story similar to the plot you are thinking of. Draco's unknown sister comes to Hogwarts and is sorted into Grffy, befriends Ginny and the Trio, and both she and Draco are hiding a dark secret... yada yada yada. So I thought I'd warn you before you go down that well traveled road.
Don't get me wrong though, your plot seems good, and if thats the way you wanna go by all means go ahead!

I'm just encouraging you to but your own original spin on it.
Your first focus must be on Antonia. You must give her some type of history. Not having a definite background planned out in the begining could make your plot unstable and give it a few holes.
Y'know what I mean, have certain things already down for Antonia. Such as, 'Where did she come from?', was she in a private school or instiution for all girl witches? Or is she a squib and spent her life living in Malfoy Manor, or some far-away boarding school so she couldn't cause Lucius shame. How old is she? And why is she at Hogwarts? And for how long? Because, I doubt a squib would feel comfortable around alot of witches and wizards, especially if that squib is a Malfoy, there is a reputation and a noble name to uphold. (But hey its your story you do it your way!

haha)
Also, you have to decide on what type of personality Antonia is going to have.
Is she going to be the sly cunning slytherin in sheeps clothing, a good girl, or mix of both. And if you decide she isn't a squib and you want her to attend Hogwarts as a student what house should she be sorted into? Naturally people would only choose between Slytherin of Gryffindor, however there are 2 other choices. It quite common for people to place their extra Malfoy member into Gryffindor house, have the extra Malfoy befriend the trio, fall in love with Harry, Ron, or Hermione (depending on the sex of course..haha) and become fast and best friends with Ginny which usually causes Lucius to go insane. And then on the other side of the specturm, the extra Malfoy is sorted into Slytherin but hangs out mostly with Gryffindors(namely the Trio and Ginny), which causes angry house rivalry and livid Lucius. (haha Livid Lucius.. that sounds funny!)
But it doesn't really matter what house shes in, if u decides to sort her that is, he depends on the personality. Does she care for Draco, cas she see through the bad-boy exterior he puts on day by day and really reach him. (Not in a sappy way, but since they are sister and brother and had to grow up in same way in the samw house she could be the one he REALLY talks to, he can't really talk to those brainless dolts Crabbe and Goyle. And once a again I dont mean in a sappy way I mean from anything to Quidditch to the next Transfiguation essay he has). And if they have this connection she could she that he has feelings for Ginny and... well... casue mischief *wink wink* haha!
I dont know how to tie Ginny into all of this yet. She and Antonia could be friends, or she and Ginny could be they same age or something. And depending on when you set this fic, Ginny could be the prefect assigned the job of excorting Antonia around, and thats how their friendship coulld start. Well, thats enough from Sassy! haha
Whoa! I wrote alot! Naturally that was just advice, so if this displeases you, you can disregard all of it! haha. If ya need anymore help. You can IM me, PM me, or email me! (Haha I feel like kim possible!...
Call me, beep me if ya wanna reach me 
Haha enough of my rambling! Bye! *waves*