Part of the reason they feel the need to "act" normal is because they are not normal. They are extremely gifted intellectually and people with higher IQs simply do not see the world or perceive the events in the world with the same perception and level of understanding as people with lower IQs do. Genius is usually considered to be an IQ of 160 or higher. Gifted is 130 and higher. "Normal" is considered to be 110-129. Less than 70 is considered to be learning disabled.
It is safe to assume that these siblings (and others in their family since giftedness usually runs in the family) have at least 160 IQ since they are time traveling and studying the physics and science concepts that make this sort of travel possible.
If you have ever been identified as gifted or have known someone in the class for the gifted, you might be aware that they think differently, they have a different set of priorities or things they think are important, and that others look at them as if the "gifted" are strange, weird, or a bit off. The classes for the gifted are smaller than regular education courses since less than 10% of the population has an IQ this high. They get to do super cool things because the regular stuff (which give the normal kids fits) bores them--they already know it and need a heavier challenge. Therefore, smaller classes and cooler stuff means that others--students and teachers alike--are jealous of the kids and the teachers who teach them. They feel like outcasts, but they are regular kids who just want to be liked and accepted like everyone else.
Thus, we're back to your original question. Meg and Charles "pretend" to be normal so that others will not treat them like weirdos and outcasts. So that others will include them...as all kids want to be part of the "in" crowd.