Want a Guinea Pig(s) Convincing Parents!!!

So I have two beautiful girls, Cola and Pepsi, but I have always liked to have more than two. One of my mums friends is a vet and last year they decided to breed two guinea pigs. Unfortunately they live quite far away and my mum doesn't talk to her very much, anyway last year we were getting some more guinea pigs after my two boars, coco and pops, died and we were thinking about getting some off my mums friend, but they had all gone by the time mum had got around to asking so we got Cola and Pepsi from somewhere else. Last night my mum saw her friend again at a party and asked her about the guinea pigs and my mums friend said that they were breeding again in the summer and offered my mum some, but my mom said no. Of course I was devastated when my mum told me and I'm trying to convince her to let me have some. I'm not sure what the problem is as we have a large spare hutch and a big garden so space isn't an issue. Mum said it was partly because we didn't have enough inside space as we keep Cola and Pepsi in a small cage in the hall when it gets cold outside, but we have a big shed that we used to put my late guinea pigs in when it got really cold because they lived outside all the time and were happy. She knows I can take the responsibility because I do everything for Cola and Pepsi without moaning and I think the problem is she just doesn't want some more , but I feel like its partly my decision as I will be looking after them and I know I can take the responsibility. So i need some opinions and ways of convincing her. One last thing! My mum is one if the most stubborn people I know, just behind my dad! Thanks!