Family in the wedding party

Hi, I've recently gotten engaged and I'm trying to decide on bridesmaids. I've already decided on 4, with three being my best friends and one being my sister but I'm feeling the pressure from my family and I feel really guilty about my future sister-in-law. A few days following the announcement of my engagement, my uncle spoke to my mum assuming that I was getting married by the end of summer or at the very least within the year. When my mum put him right and told him that it was going to be a long engagement so that we could put some money aside for the wedding, he said that it wouldn't be great for my cousin as she would be around 15 by the time that we have the wedding, which according to him 'takes away the childhood magic of being a bridesmaid'. The problem is that I never had any intention of making her a bridesmaid, and if I were to make her one, then I would need to make my other cousin/her sister one as well. I don't want too many bridesmaids and I'm already happy with the number that I've already settled on, but I now feel bad if I don't make her one as apparently she is really excited to be one (which is to be expected as she's only 10). In addition to this, I also feel guilty about not making my future SIL a bridesmaid. Luckily my fiance's family haven't shown any expectations yet of her being one, but I feel that inevitably there will be some pressure in the future. I'm making my own sister a bridesmaid and have asked my fiance if my brother can be a groomsman, and as a courtesy to my fiance, asked if he wanted me to make his sister a bridesmaid. He's been very firm on his decision that he doesn't want her to be one as he's never particularly liked her, and I'd like to respect his decision, but I do feel that if there is any upset over this decision, it will be directed towards myself and be viewed as though it is my decision. Has anyone got any advice on how to avoid any anger from family members over people not being invited to be part of the bridal party? I appreciate that emotion isn't always avoidable, but any advice on at least letting people down easy, would be appreciated