Need birthday gift ideas ! Help Please!

I agree with mrsswns, a pandora bracelet with a few charms would be great!

If you don't like that idea you could get her the Tiffany Bead Earrings (the 8mm are $120 and the 10mm are $150). They are classic and can be worn forever! Also she could eventually get the bead bracelet to match! http://www.tiffany.ca/Shopping/Item...+-r+-x+-n+6-ri+-ni+0-t+bead+earrings&search=1

You could also get her the Return to Tiffany Bead bracelet with a small charm on it. Those are $150 as well. http://www.tiffany.ca/Shopping/Item...+-r+-x+-n+6-ri+-ni+0-t+bead+bracelet&search=1

There is also the bean pendant which is $120 for 9mm or $180 for 12mm. I personally have the 12mm and it is the perfect size. http://www.tiffany.ca/Shopping/item...-t&cid=288158&selectedsku=25185137&fromgrid=1

There are various Tiffany silver rings under $200 you could choose from.

Another cute pendant is the bow pendant. It is $140 for the mini size. http://www.tiffany.ca/Shopping/Item...2-p+9-c+288158-r+501323338-x+-n+6-ri+-ni+0-t+

There is also the Sweetie Bracelet by Links of London. It is $220, so a bit more then you were looking to spend. But it is a great bracelet.

It all depends on what you want to get her. Let me know if you need any more ideas, I have plenty! Are you looking to get a bracelet, necklace or earrings? I can think of more if you need more!
 
I would def stick with tiffany! What about a sterling silver key there are some beautiful ones for 165, or a pendant would be nice, and she can use one of her own chains with it.
 
Pandora may be more personal, but at her sister's age it's all about status (unfortunately) so Tiffany's would provide that status and be more recognizable than Pandora, I would think.
 
I think a Tiffany key is an excellent idea. I'm 25 and (no offense to anyone) I'm not very excited about the "look" of Pandora and would much prefer something from Tiffany. Also, an item from Tiffany is a complete look, whereas the Pandora bracelet means that she will have to buy more charms if she wants to fill out the bracelet.
 
I was going to suggest a Pandora bracelet with a charm but I see many already have!

I would have loved to have been given one at the age of 20 and I know many young people who would also love one.

But only you know the 20 year old in question and if you think a Tiffany piece is what she will adore then go for it.