I have an 8 yo standardbred mare who I broke in 4 years ago with the help of an experienced friend. For the past 4 years, I'm the only one who's ridden my horse. In those four years, she has only had two rides on her by other people, and these were only short (10 mins) by experienced riders.
Anyway, last weekend, I put my sister on my horse so she could have a ride. My sister has been riding for almost as long as me. Then my horse put her head up really high, looked really worried and refused to move until I lead her forward. She was really tense and had her ears turned back, facing my sister (not grumpy back, just listening). Whenever my sister, squeezed her to get her to go forwards, my horse stopped and looked really unsure of what to do. She wasn't being naughty, she's usually really calm and compliant.
Is this normal for a horse to act this way if it has only had one rider since it has been broken in? And why does my horse do this???