regarding family visit visa

I am on student visa in UK which is going to expire on 30/09/2014. I want to invite my brother ,sister in law and their 9 months old son to UK for my graduation ceremony which is on 24/07/2014 for family visit visa from Nepal. Can they apply yeah because my visa is going to expire in 4 and half months? I went to Nepalese Vfs website and filled the application form online but the confusion is how do my sister in law and son can apply to UK, will they be applying in the same application with my brother or his wife need to fill the application form separately? And how about the son, I tried looking online but couldn't find any information relating this.
I am going to sponsor them so how much money do I need to show in my bank account, and do I need to send them the other supporting documents like payslip, bank statement, employment letter original or the photo copy will do? And how many months bank statements and payslips do I need to show?

Please advise.
Thank you
Regards