EDIT: reason to be offended?

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My brother has his friends over every Sunday, which I don't have a problem with, but there is one girl I don't like.
First time I met Maddie was last weekend. I have a habit of getting up at 10:30 on Sunday mornings, so as normal, I walked into the kitchen to make brekky, and I found her standing there looking in my fridge. She looked at me, then walked back to my brother and his friends.
This morning, she came in and didn't even greet me when I was in plain sight. I told my brother I thought she was rude, and he blamed it on her hangover. 10 minutes ago, I was folding laundry on the dining room table, and she was in the kitchen with my brother (right next to the dining room and there's no door either) and once again she kept looking at me, but didn't make an effort to greet me.
Is this a valid reason not to like someone? My boyfriend says I'm too picky with who I like and don't like. All my brother's friends except her make an effort to greet me and my mother, and I'm a strong believer in greeting the host & their family before anyone else.
**EDIT**
Thanks for all the feedback guys! 
I should've said before I do have social anxiety, and I don't have great social skills, and I'd understand if it was social anxiety with her, but it doesn't seem to be that way. She had never met 3 of my brothers friends that were over yesterday, but seemed extremely comfortable with them, and she did say hi to me once prompted by my brother, 3 hours after I brought it up with him.
**EDIT #2**
So I came back from an event last night, and found her again, sitting on the couch with my brother. I greeted them both, only having my brother respond, and went to make myself some dinner. She came into the kitchen, helped herself to whiskey, and left. Didn't say a word to me all night. I GREETED HER AND SHE DIDN'T EVEN ACKNOWLEDGE ME. What a rude little cow 😤😤😤