no but yes. it's not possible to invite people to your town both locally and online since you can only have your gates open to either local or online friends. but, what you can do is open your gates online & have both online and local players visit you online.
if paying for wifi connection is the problem then you won't be able to play with your sister and online friends at the same time until she has a wifi plan. :( my family ran into the same issue, so you should probably buy a family wifi plan and add your sister to it.