I also have an identical twin who does not stutter, although we both developed stuttering when we were around 3-4, he grew out of it (with some speech therapy) and I didn't. I honestly think a significant reason of why I stutter is because we were always compared to each other, and for childen learning how to speak, one twin might speak more/better than the other, and it would become competitive ( from both child's and parents' perspectives ). I also have two friends who are non-identical twins, and they both have a mild stutter. That's why I strongly believe that parents comparing their speech in early development "caused" their stutter. It might be that you are taking your twin's speech as your reference for what you (physically) can be, and it can be frustrating, so stop comparing yourself to others especially to your twin, it won't benefit you in any way.