Annie and Bettie are close relatives and live a few miles part.
When Annie is out doing errands, she will sometimes drop in to see Bettie unannounced. Annie does the same with Bettie, so calling round unannounced is not a problem.
Annie called round to see Bettie, but nobody answered the door. She went round the back of the house into the garden to see if Bettie was in but had not heard the bell. Annie feels that as they are close family it is reasonable to go round the back to check if they have not heard the doorbell.
Bettie feels that if they do not answer the door then Annie should know they are either not in or do not want guests and that going round the back is intrusive.
YABU - Annie is right. Peering though windows at the back it out of order.
YANBU - Bettie is right. They are close family and she thought Bettie hadn't heard her at the door.