Not in a poly situation but I have experienced something similar recently.
A family member of mine had a long relationship with a really lovely girl. After they split up, they remained close friends and we all hoped they'd get back together again.
Anyway - they didn't and eventually, my family member turned up with another girlfriend. I had the same feelings of not being that keen to meet her, a bit guilty about maybe liking her, worried about what the effect would be if I didn't like her (she's very keen to be part of our family and very upfront that she's looking for a marriage and kids).
I have met her now and I really like her. She's just lovely - as lovely as the last girlfriend. She lives close to me and we are becoming friends.
My only real sadness about the whole situation is new gf feels very threatened by the last gf - totally the fault of my family member! His handling of the situation sucks.
(I quietly hope that she ends things with my family member and finds somebody who is as keen to marry and have children as she is - she and I can be friends regardless of whether or not she is in a relationship with a member of my family).