Back in my 20s, after 6 years of part-time study whilst doing a full-time job, I qualified as an Accountant. As planned, I then had my first baby and to help the family budget I did the accounts of sole traders, at home, juggling it around childcare, housework and baby naps. My mother never agreed that I was "working" because I did it at home.
The tax office didn't agree with her, neither did the National Insurance folk, nor the Professional Indemnity Insurance people.
When I asked her to have my daughter now and again she refused, saying if I was at home I could look after her myself!
Later I got a "job" as an Examiner for a Professional Accountancy body which I also did at home. My mother didn't think that was a job either, even though over the following 25 years it earned me today's equivalent of about £25k a year. The tax office took its cut of that as well.
When my mum had to go into a care home at the end of her life, guess who paid the top-up fees for her (and willingly)?
You will never win with your sister, and as others have said, make a light-hearted joke of it and enjoy your lovely job.