My mom seems to care more about my little sisters friends rather than me. (Advice please)

So I’m currently on vacation. And my mom decided to have my little sister bring 2 of her friends. But I just find her friends a little sketchy tbh. I’m going to start off with my mom and step dad are pretty well off. So they have money to burn and what not.... I guess....Anyways with that being said my little sisters friends have my mom buy them shit! It’s not even things that they need it’s like she’ll buy them make up, eye lashes and other things. The only reason I’m livid is because 1.) These friends have been horrible influences on my little sister. They have her over here taking shots and smoking weed, my little sister barely turned 14! 2.) While these little snobs were in my moms class they got pissed off at her because she told them to get to work. But instead of doing their work like normal students they kicked her chairs and said “this is fucking stupid and left.” And 3.) I’m just fucking tired of my mom putting these kids before me. Like I get it I’m an adult but when ever I need something, need a favor or simple “motherly advice” She would make up an excuse. But when it’s them she’ll bend over backwards for those girls. Idk these girls seem suspicious to me I feel like their only friends with my little sister simply because of fancy vacations and having someone else’s mom buy them shit. I don’t know ladies I just need some advice because when I vented to my bf he got upset with me and said I’m just starting shit. Have you guys been through this? I only posted it here because she’s always found someone to put before me and it’s caused me to have a lot of mental issues. I just have no one to go to for it. Because my mom doesn’t believe there’s anything wrong with me. :(