If my sister-in-law has been in abusive relationships and is now thinking about taking my niece out of the country, is there anything we as family can do to prevent her from doing that?

You have a couple of different options.  One is to call CPS and hope that they get around to doing something about it.  Some local branches are better than others. 
If you have a slow moving CPS group, then you can potentially take matters into your own hands.  If both parents are a danger to your neice, then you can file a suit affecting parent child relationship seeking custody of her.  She would need to have been in your possession for a period of time.  The six months doesn't have to be continuous-- just total.  If you don't quite hae the six months, then you might want to encourage your sister to go, but leave your neice with you so that you can obtain the six month mark.  I know it's horrible to trick your sister-in-law, but it appears that she's not in a good place to help herself or her daughter. 
Another option, is to simply see if she will agree to give you temporary conservatorship.  Consent orders are always easier to obtain via legal paperwork, but they can still cause hard feelings. 
A third option which can be mixed with either of these two options is to get your sister-in-law to file for protection for herself.  Until she stands to protect herself, she will always be running.  This is not a healthy lifestyle for her or her child.
To know exactly which option is best for your situation, consider setting up a consultation with a family law attorney.  Many are now offering free consultations.  They may be able to give you additional options depending on some other circumstances of your situation.