(Mainly spoilers for Omen of the Stars)

Being jealous doesn't excuse Blossomfall's actions and definitely doesn't make her hating Briarlight all right. Whenever someone got more attention than her or she didn't get what she wanted in general, she always acted rude to that cat. Now in The Forgotten Warrior she acted like a jerk towards Dovewing because Bumblestripe wanted to spend time with a friend. Not to mention, it was completely uncalled for, especially when Dovewing did nothing to her ever.

"Blossomfall's ears shot straight up in surprise. "Excuse me, but who's your littermate?" she muttered. "You could show a little loyalty, you know." - The Forgotten Warrior, page 198

Notice she called Bumblestripe disloyal to her for wanting to spend time with someone else. Blossomfall was pulling the guilt trip on her brother because she didn't like him wanting to be with someone else. She has always been like this. This didn't involve anything to do with Millie. After their training session was over, Blossomfall criticises Dovewing for Bumblestripe pairing with her:

"Bumblestripe was fine, but Dovewing, you need to do a lot more work," she [Blossomfall] meowed. "Your paws were all over the place. And you seem to have forgotten that you have a tail. It's there for balance, you know." - The Forgotten Warrior, page 200.

It's not a crime to hang about with someone else. If Bumblestripe was restricted to just being with Blossomfall all the time, like she seemed to expect from him before going in a huff, he'd get fed up of her.

Before this, in Sign of the Moon when Bumblestripe was with Dovewing, Blossomfall says this. Again, she takes out the guilt trip on Dovewing this time.

"You know, you've stolen my brother's heart," the young tortoiseshell warrior murmured. Her [Blossomfall] tone was teasing, but the gaze she turned on Dovewing was serious." - Sign of the Moon, page 81.

She also treated Ivypaw badly in Fading Echoes because she wanted to go with Dovepaw but wasn't paired with her:

"No way!" Blossompaw's horrified mew took Ivypaw by surprise. "She's not properly trained! Can't I have Dovepaw? At least she can hunt."

Ivypaw glared at the tortoiseshell-and-white she-cat. "I know how to hunt!" Hawkfrost had just taught her two new moves!

"You've hardly caught more than a mouse!" Blossompaw complained. "Dovepaw's brilliant! She can hear prey anywhere!" - Fading Echoes, pages 124-125

Blossompaw was waiting on the wall surrounding the tumbledown nest. "You can't even keep up," she scoffed.

"We're supposed to be showing them we can work together," Ivypaw snapped.

"Like I'm going to give you the chance to hold me back." Blossompaw jumped down from the wall and headed past the plants Jayfeather carefully nurtured. The scent if them made Ivypaw's mouth water, but she knew the warning given to every cat. Stay away from the catmint. It was the only cure for greencough, and more precious than poppy seeds.

As she disappeared around the corner of the wall, Blossompaw called over her shoulder, "Just stay out of my way!" - Fading Echoes, pages 126-127

"What did you have to show me up for?" Blossompaw hissed as soon as thry were out of earshot. "This is my assessment, remember?" - Fading Echoes, page 129

"Fox-dung!" She cursed through a mouthful of feathers. "One wretched bird while you have to catch a squirrel!" She glared at Ivypaw. "If I fail this assessment, it's all your fault." - Fading Echoes, page 132

Her treatment to Ivypaw was completely uncalled for however she has always acted like this. Briarlight wasn't even injured at the time. People seem to forget Blossomfall's attitude has always been like this even before Briarlight was crippled.

Blossomfall's different. She actually knew how dangerous it was yet still went. She never had a problem until SOTM but after that she never moans or self-pities and doesn't even care. And if she did have a problem, she could have spoken to someone, Millie or Graystripe. But instead she acted woeful and confessed she hated her sister.

"No. A good cat wouldn't be jealous of an injured littermate. So that's why I've ended up in the Dark Forest. I'm not stupid. I know it's where cats go if they're not allowed into StarClan. But I guess I won't get into StarClan either, because I hate my sister for being injured. So the Dark Forest is where I fit in, and I'm getting good training, better than anything we get here." - Sign of the Moon, page 246

There is no proof Blossomfall was forced to go into the Dark Forest. She decided to go there just because her Clan's praise wasn't good enough for her, and she could only focus on her mother's attention rather than being a decent warrior to the Clan. Blossomfall didn't need it, she WANTED it. She can make her own chocies and it is ultimately her own fault for going to the Dark Forest not Millie's. Since when did Millie have powers to control Blossomfall's choices? Now Blossomfall only joined for attention and not to be a better warrior for her Clan. The only reason she stated was for attention, not a better warrior. People expect Millie to know how Blossomfall feels but how could she if Blossomfall never says anything to her or even bothers to care and visit Briarlight? There was only one scene in Fading Echoes she's with Briarlight. Just one.

Millie only scolded Blossomfall once, not a million times like some people make out. It's not like Blossomfall did any good when she was in the tunnels. In fact, it was her idea, and she did waste an entire day when she should have been hunting. She almost got herself killed because of her curiosity.

"A weird feeling crept over Ivypool as she gazed down into the hole. She felt strangely reluctant to go near it. "What about hunting?"

"We can hunt later," Blossomfall replied, her eyes sparkling. Fueled by excitement, she seemed to have thrown off her earlier exhaustion. "Let's explore!" - Sign of the Moon, page 216-217

Blossomfall wasn't behaving like a proper warrior. Every warrior would have enough sense to understand how dangerous and off limits the tunnels were. She seemed to have forgotten about what everyone thought happened to Hollyleaf. The tunnels could collapse any second.

Bumblestripe made an effort to comfort Briarlight and at least listen to her. He could understand Millie and is no older or younger than Blossomfall. Blossomfall didn't. If anything, she stayed away from Briarlight. Blossomfall could have helped Briarlight and understand her condition rather than acting as if Briarlight's condition was no big deal. The world does not revolve around Blossomfall. It's not like it's Briarlight's legs that are crippled, it's her whole spine.