Ok, older female here, lots of work experience and maybe some advice on how you might solve the situation;
First, read this article. It's about emotional intelligence in the workplace (yeah, sounds mumbo-jumboish but do me a favor and read it since it's pretty short though the concepts aren't.)
http://www.fastcompany.com/3028712/7-habits-of-highly-emotionally-intelligent-people Secondly, it sounds as though you have a long-standing habit - well both of you do - she butts into your job and you get annoyed. Fixing it won't happen overnight - just just keep that in mind.
Thirdly, figure out exactly what you want to accomplish;
1) Job duties (do we all pull together or are our duties our responsibilities?)
2) Effective communication - how can we better get along when there is conflict?
3) Miscellaneous stuff like mail, shipping - should she open everything, stamp the date recv'd and deliver to the correct person? Should she be responsible for the returns of items like you mentioned and if so where can she find the necessary info so it doesn't get lost?
4) How can you personally change how you are treated - are there things you do that don't serve you in terms of being treated like an adult? Is there a way to communicate differently with her that might change her mind? Do you treat her as an equal partner in getting the job done? etc... introspective stuff...
As to the" how you are", consider if there are any changes you can make - even a good morning smile and a genuine hello can begin to melt even the frostiest of hearts. Ask for her help - say "I think they want the label on the outside but maybe you could call for me and check so we don't get this returned?" Actually she sounds bored and is looking for things to do - let her help with administrative stuff. She'll feel useful and you'll have less paperwork to do.
If that doesn't work after you've made a concerted effort then try this...
Make a list of solutions for the on-the-job stuff 1, 2 and 3... Then present your solutions to her. If that doesn't work then try this next thing;
Fourth, ask for a office meeting. Include your boss, you father and the office manager/receptionist in your invitation. Before you invite them, I'd ask you to prepare a few things (pads of paper, pens, maybe a white board). Write a short presentation about making the office more efficient and more pleasant- i.e., assigning specific tasks to one person, arranging a monthly lunch (or even a bi-annual company outing) with everyone to increase communication, and cooperation. Basically what you want is a friendly, more effective work environment.
When you send you invitation make sure you let everyone know that this is a brain-storming session to resolve some management issues and will only take 1/2 to 3/4 of an hour. Make sure you don't go over that.
The thing is, your presentation has to be somewhat formal, show that you know how to run a meeting and is about everyone working together - not nitpicking about her opening your mail. It needs to show you in an adult light as a responsible member of the staff.