Difficulties with advisor inattention during MSc
don't say "they don't believe in my ability as a researcher", it's self loathe and may become a self-fulfilling prophecy. As a supervisor of msc and phds myself, I think research skills can be learned. Are they pointing you towards resources at the university, like training on research methods? Do you think you lack those? 

I add that weekly meetings are a luxury at my university, so maybe your supervision is working fine, and you may have just a bit of envy of the extra attention your peers receive? Or do you think your research is really suffering and struggling to progress?

I saw you've been thinking about possible reasons above, but I believe there are more explanations like (4), maybe the subject area of the other students is closer to what the supervisor does, so they have more to talk; (5) your area is more advanced than what they do so they don't know the methods and don't have much to contribute. I believe that you can talk to the main adviser openly about your concerns. You can directly ask them what they think about your research and where it's going. You can also ask about the frequency of the meetings. It will be a delicate conversation but it's better said and resolved than let accumulate. But the main question is, do you believe it's a good project? Is it interesting and innovative? Are you on top of the literature? 

And grab the opportunity of writing that a publication, obviously it's very good for your phd application but also can increase the interest and frequency of communications with the current advisers.