H1 Expired and Travelling with H4 and then filing H1 Extension

[balak]

Hi,

My sister was in US with H1 status for 2 years from 2013 to 2015 then she moved back to India. Now (2017) her husband came to US with H1 Visa. My question is:

Whether she can come to US with H4 Visa and then apply for H1 extension?
Her company is not currently filing H1 extension and she is afraid whether H1 visa extension will not be possible if she moves to US with H4 visa.