Hi, I'd like to help you with your question.
Yes, it would be fine to contact your niece and nephew. You may want to have a general idea of what you would like to say to them, even write it out if that helps. I would keep your tone soft and non threatening. Let them know because of their situation growing up you could not have regular contact with them but now you would love to have contact. You may or may not want to let them know you tried contact before, depending on their responses to your contact. You may also want to save more information until later, if they want to continue the relationship with you.
If they seem unsettled by the contact, keep your conversation brief and let them know that you welcome them contacting you when they are ready. They may have been told things about your family because of your brother's actions, so be prepared for the possibility of hostility or misconceptions about who you are as a person.
If they want contact, go slowly in building the relationship. Stay away from any topic that could cause conflict, like your brother and their childhood. Answer any questions you feel comfortable answering, but try to respond as a neutral party. In other words, don't become emotional and interject your opinion in your responses. You want to build a strong relationship with them before you express your opinion on any of the issues in your family.
If you contact them, you may want to try to do it by phone or even in person. Facebook is great for friends and family who you are friendly with, but this type of contact should be in "person".
I hope this helps you,