Translation:My sister is helping me to make lunch.
Both your and default translations are true, but usually Russian sentences in present time should be translated into English using Present Indefinite time. It can be translated using Present Continuous when there is specific markers in the sentence (i.e. сейчас/now or как раз/currently) or if it implied in the previous/following text. So in this case "My sister helps me" is more probable then "My sister is helping me", but both can be used
Two verbs can be placed back to back in Russian. Some examples you may have already encountered at Duolingo include:
Я хочу есть.
I am hungry.
Я хочу пить.
I am thirsty.
Я хочу спать.
I want to sleep.
These are simple examples with one subject and one verbal phrase. I think you may be suggesting that placing "мне" in between "помогает" and "готовит" is following some standard, routine convention for such sentence constructs. I don't know that that is the case. For example, if I were to write:
Моя сестра мне помогает готовить обед.
my issue wouldn't be with the back to back verbs, which are really just one verbal phrase -- a conjugated verb and an infinitive. What seems odd to me about that order is having the indirect object immediately follow the subject. But a page on MasterRussian.com claims that the sentence, "A cat caught a mouse" can be written in six different ways, one of which includes the object immediately following the subject (and another version listed even precedes it!). However, I do think some word order is probably more natural than others. And it doesn't take long before something just doesn't look right or sound right to you as you learn a language.
Speaking of Russian word order, duolingopk1, you're correct, all talk about the flexibility of the Russian language aside, the pronoun often does precede the verb when it is a subject pronoun, but "мне" is in dative case which corresponds with the indirect object in English. It is my understanding (and observation) that the object often comes after the verb in Russian even when that object is a pronoun. In the post titled, "A guide to the Russian word order" by Duolingo user szeraja_zhaba the following is written:
Objects usually follow the verb: я ви́жу соба́ку 'I see a dog', я понима́ю грамма́тику 'I understand the grammar'.
But this follows it:
But when object is a pronoun, it usually precedes the verb: я его зна́ю 'I know him', я ничего́ не ви́жу 'I see nothing'.
If this is something you read, I can totally understand why you brought this issue up. I'm not going to question szeraja_zhaba. I don't know much about him (her?), but I've read a lot of his/her comments and have found them to be knowledgeable and valuable. Having said that, you may want to take a look at this video here:
I don't know if that video addresses your specific question(s), but this thread:
Specifically states that generally the order is "SVO" or :
Subject - Verb - Object