OK, looking from your sister's perspective, you appear to be saying:

I am a midwife. I know all about babies. I am further on in my pergnancy than you. Everything that happens to you in this pregnancy, I have experienced first, either from this pregnancy, a previous one or as a midwife. I know everything. You are wrong about pregnancy things, such as what your GP should be doing. I trump you.

What does your sister do? Do you value her career? Her opinions?

Could be that I like being the person with the knowledge and don't like being told otherwise, when I will contradict people. Especially if they tell me that MY knowledge of MY body and MY pregnancy is wrong! Had enough of that last time.

Also, as you are a midwife - I have had more than enough of midwives telling me I'm wrong. And that was before they messed with my birthing mojo. Has she had bad experiences with midwives before to really, really resent your knowledgeable midwife hat going on, even though you are doing it through love?