She is above 21 years of age, married and with children.
Perhaps, if she has or is believed to have immigrant intent for the purposes of a visit, then the visa may be cancelled. These sorts of questions are best answered in detail with a competent and experienced immigration and visa attorney. Good luck.
This is general information, not legal advice, and does not create an attorney client relationship.
She should still be able to use her B2 visa while her petition is pending. She should always answer any questions truthfully, and whenever she visits the US she should not stay for longer than the authorized period on her I-94. Petitions for siblings and married sons/daughters take quite some time, and she is lucky to already have a B2 to be able to come and visit her relatives in the meantime. She should use it wisely.
[This answer is for general purposes only and does not establish an attorney-client relationship.]
She probably won't 'lose' the visa ... but they might not let her into the US.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.