You are actually wrong in this context since Hindi does distinguish between the tenses. The use of हैं in the end implies use of present perfect tense(has come) while without it, the sentence would translate to simple past tense(came). eg. मैंने आम खाए | (I ate mangoes) vs. मैंने आम खाए हैं | (I have eaten mangoes)
अंग्रेज़ी (English): uncle, mother's brother, en:uncle
फ्रांसीसी (French): oncle, frère de la mère, पु. fr:oncle
with हैं at the end it's the Present Perfect tense http://www.learning-hindi.com/post/5457387996/lesson-110-the-present-perfect-tense
Peter's maternal grandmother will be translated in hindi to "Peter's Nani". As already explained "Mama"= Mother's brother or Peter's Uncle (X's uncle). Incidentally Chacha or Taya in hinidi is also translated to Uncle in Hindi. Chacha is father's younger brother and Taya is father's older brother. But all (mama, chacha, taya) are Uncle in english.