I doubt you should do anything to alter your Mom's quilt and all of the suggestions you have received so far are good ones.
Just in case you would like to know ... I have had to continually add borders to enlarge quilts and it has always worked fine. Usually someone will have a baby quilt that needs to be a toddler quilt that has to turn into a big quilt. Or they buy a quilt some place and then have a California King size bed, so I add to it. It works, but it is not easy ...
Not sure how your Mom made your quilt and if it is hand-quilted or machine-quilted. I would not add to a hand-quilted quilt usually though.