If you only want to stay for two more months,you can file the application as
late as possible (eg. mid March,check the immigration web for the latest
information).Because once your application is received, you are legal to
stay in the US until you get the final decision. That means, if you haven't
receive any decision in April, your mom is still legal to stay. Most likely
you will not get any decision in 2 months and your mom is free to leave in
June even if you haven't gotten any final decision yet.If the application is
denied, make sure your mom keep the deny notice and the application receipt
(evident for her next visit.Because she will need to explain she is legal
to stay for more than 6 months.