In-law drama

So me and my fiancé booked our wedding a year ago and now his younger sister has decided to book hers. However our wedding is in October this year and hers is end of May. Our families were excited for our wedding and now my partners family seem to have forgotten bout ours and are only talking about his sisters wedding. My problem is is that his mum paid for his older sisters wedding dress and his dad is paying for his younger sisters venue. But neither have offered anything for our wedding. I don’t expect it but I just don’t think it’s fair. I also have been asked by my partner to get both his sisters involved in our wedding prep but I wasn’t invited to be involved in his older sisters wedding and now his younger sisters wedding. However my big problem is is that his younger sister is asking to use all of our wedding stuff such as chair covers. How do I tell them politely to not ask and appreciate that we have planned our wedding and that it’s not right to ask us to use our brand new stuff before our own wedding?