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(I am not a linguist, so I don't know proper terminology)
When studying Spanish and French, I quickly learned that many very common verbs have irregular forms; the reason given was that common usage gave way to more possibilities for the word to change.
Why then is the word "brother" (same for "father" and "mother", etc), such a stable word in Indo-European languages, given that it is clearly a very common noun?
Common usage creates irregularity, but not because those words are likely to change—rather, it's because they're not likely to change! These common words are likely to stay in their current forms even as the rest of the language changes around them, leaving them as irregularities to be memorized.
The way you conjugate verbs in English looks quite different from Spanish and French, for example. But English "is", Spanish es, and French est look quite similar (and quite different from how verbs work in the rest of the language). This particular form goes back to *h₁es-ti, a perfectly normal way of conjugating the verb in Proto-Indo-European, which has stuck around in various descendants despite the whole system of verb conjugation evolving in new directions. (Similarly Russian jest', Hittite ēszi, Sanskrit asti, Persian ast…)
