My cat hissing in her harness

Hi guys! In the last two years, I’ve gotten two cats. I’m fairly new to cats as my first kitten was my first experience, I’m learning a lot, but need some help.

My older cat female, roughly 2 years old, loves to go outside and she has a harness that I put her in with a lead and she sits out in the backyard with me. As the weather’s gotten warmer, I just started taking her out again the first few times were fine. Her younger brother, our new kitten ,male about one year-old, outgrew his kitten harness, so one day I put him in her harness and took him out. Now today when I tried to take her out, she was hissing and won’t let me touch her after being in the harness.

I can’t get the harness off of her because of her hissing and running away and hiding, my question is, could she be doing this because he wore the harness? ( like a territorial thing) or is the most likely culprit that the harness might be too tight? He kept slipping out of it so I tightened it when he was wearing it and for her I just put it on, but she won’t let me close enough to check. Before this, she’s had no issue with her harness. I also thought maybe some noise outside spooked her but it’s been about two hours and she still won’t let me touch her. I figured after being inside she would’ve calmed down.

Please help, what is the most likely situation for her acting this way.