Any one can be representative in family member. So if you want to become representative , You must sign on page # 2 section 8 . Your Representative Declaration . and provide the date too. Your inlaws also must sign page # 3 ( Signature of applicant ) and Signature of Spouse ( Your mother in law )
For this form do the hand signature , because in the past they have rejected some form and asked for hand signature.
In my case i send page 3 to my mom and dad , they sign it and scan it back then i attached that page to page 1 and 2 and send it via Web Form .
I hope this information helps ? If you are still unsure how to fill out the form i would recommend to check some YouTube video , there are some very detailed videos people have posted