Found another workaround for iBooks crash issue, apparently it works for my sister's iPad, finally. So, I'll be summarizing two solutions into one post.
Try at your own risk.
Solution 1 - iDonnie from Macrumors.com0. Delete iBooks and/or every other patch/patches on your iDevice.
1. Install a clean iBooks from the App Store and then restart your phone.
2. Install the iBooks Fix2 from the insanely repository.
3. Respring the iDevice.
4. The Springboard now should crash and go in safe mode (i don't know why, but the only time it crashed and went in safe mode it worked for me), so if it doesn't crash go in safe mode.
5. Restart the phone.
6. Now iBooks will work (at least it worked for me).
Solution 2 - From Jailbreak.com (Make sure to have SBSettings installed first)
0. Delete iBooks and/or every other patch/patches on your iDevice.
1. Open SBSettings > Scroll down to bottom > System Wide Options > System Options > Fix User Dir Permissions > Fix Me
2. Follow step 1-5 from this link:
http://www.ijailbreak.com/how-to/fix-ibook...0-1-untethered/3. Respring and now iBooks app icon should appear on your springboard.
p/s: I don't need to go until step 6 from what shown on the site. I managed to get and throw all my PDF stuffs into iBooks and now I'm reading happily on my sister's iPad.

Actually why do we need ibook ah? If you read pdf a lot, I suggest to get Adobe Reader from app store, it's free.