Katherine, I bring you opposite viewpoint. I forced myself to breastfeed for 3/4 months and even if I think I did best thing for DS health, menawhile I caused myself a lot of pain, both physically (I got mastitis and bf was very painful when DS was 3mo) and mentally (yk, bad mother when compared to who bf for 2years, etc.).
So I agree with insisting for doing everything technically correct, I also suggest to check on you sis to avoid she gets too distressed on the topic. She will be a wonderful mom bf-ing or ff-ing, bur she may be slow to really understand that this is for real and not just a consolation prize. I felt very high pressure on bf, there is so much buzz around the topic and it seems that you couldn't consider yourself a good mum if you do not breastfeed for a couple of years. In the hormones burst after pregnancy and partum it is difficult to put things into perspective.