Your ideal BMI weight is about 167. That weight would put you right in the middle of the green section of the chart I posted previously. According to world-class running coach Jack Daniels, elite runners should weigh about 15% less than their ideal BMI weight. That would put you at about 142 lbs, assuming you are an elite runner. If you're not world-class, the you should weigh more than 142. At 110 lbs, you are severely underweight, even for an elite runner. That explains why you are so weak and why your sister can pin you so easily. You simply do not have the muscle mass you need to carry your skeletal mass. I'm not trying to be critical here. I'm just stating facts. Your problems are much bigger than a bratty sister. You are putting your health at severe risk until you put on about 30-40 lbs.
Update: Once you put on the needed weight, the problem will take care of itself. You will weigh twice what your sister weighs and she will no longer be able to pin you.
I try to gain weight, I'm eating 4 meals a day but it never really works. We went on holiday last summer and we ate so much for 2 weeks. I lost 2 lbs at the end of it.