Is my step mum jealous of me?

i have been living with my dad and step mum for around a year now after moving out of my mums house, at first everything was great i was treated well spoke to nicely but it didn't really last for long, as she had a son herself from a different relationship and my half sister which was with my dad, i started to notice that she was always buying her son and daughter new things and leaving me out and that i would hardly get spoken to as much as the others, after a while it was getting so upsetting my girlfriend sent my dad a message explaining how i felt, he then shown my step mum and we all have a chat and it seemed all right again for about 2 weeks and this time it really got worse, when my dad was at work i would somehow always be doing something wrong like not dusting my room 1 day or getting up a bit late as i had just finished school, so she would ring my dad up so he would have a word with me. it happened so many times it was horrible and the arguments they kept having were getting worse and becoming more often, my dad slowly got closer too me and told me what they were about.. as i was getting a car at the time my dad bought me some new alloys and she went in such a strop because he had bought me something and not her.. so she went out and bought a new TV for the kitchen (pathetic i know) as the days went on afterwards she would hardly talk to me.. and walk out the room if i walked in it.. it was so like she didn't want me there but i wasn't going to do anything just then so i stayed quite. And also before my dad met her she lived in a run down council house but now shes got 2 houses nice cars lovely furniture and she turns into a complete spoilt cow.. i hardly ask for anything unless i really need it but its all too weird i cant understand why she cant be normal and like me i would do anything to help her or help anyone i just wish she was a bit nicer to me and my dad.