I like the MC/Sarah dynamic!
This absolutely fits the persona of an inexperienced young woman who is still trying to sort out her feelings.
We also have to add in Sarah's position within her family - order of birth and birth mother makes a difference in her world - so she knows she is literally seen as having less value than her other older sisters.
Sarah's self-image is shaped by her own physical attributes as well as her 'family standing' - I think it is natural for her to not understand how the MC can find something in her that she can't see in herself.
Sarah's potential to grow into a woman who values herself as much as the MC values her is part of what makes her story so special to me.
The MC gets this and being called an idiot is, IMO, part of Sarah's armor protecting her from hoping for something more.
I can imagine Sarah's mock protest and private joy when the MC gives her a matching choker letting everyone know that Sarah is one of the women chosen by the MC.
Keep up the good work with Sarah's story!!
Cheers!!