While such idiosyncrasies are what makes languages interesting, this has nothing to do with the difficulty of learning English. Why?
A new English speaker is unlikely to say “methren”. These are the mistakes they are likely to make. In context, you are going to understand these “incorrect” plurals. (And “brothers” isn’t even incorrect.) Mastering these irregular plurals is one of the least important aspects of learning English.
Pronouns tend to be irregular in any language. That’s because they’re used a lot. Which also means that they are the irregular words that students will most quickly master.
Compare this to the inflections of the Romance languages or the two syllabaries plus Kanji used to write Japanese. Although, I have found that the difficulties of different languages don’t matter much in the end. If, in addition to study, you use the language everyday, you will learn it. If you don’t use the language regularly, you will struggle no matter how “easy to learn” it may be.