"My sister is helping me to make lunch."
Translation:Моя сестра помогает мне готовить обед.
Imperfective готовить suggests that you're focusing on the process of making lunch; perfective приготовить could suggest you're more concerned with the result (maybe 'helps me get lunch made [so we can eat]). That said... when you're using an infinitive there can be overlap in the choice of aspect - sometimes either aspect can be grammatically correct, though speakers might favor one or the other given a particular context.
It's hardly a scientific study, but for what it's worth, a quick google search gave these results: помогает мне готовить - 5,590 hits помогает мне приготовить - 3670 hits
I tried checking other aspectual pairs after помогает мне and the results were pretty similar.
Just a guess, but maybe мне comes after помогаешь here so that it can be next to готовить, since it's мне who is the primary person doing the готовитьing. In other words, мне is the indirect object of ты помогаешь, but it's also sort of the subject of the verb готовить in the "to cook lunch" clause.
I am finding the same thing. And that is 3 years after your comment! So, I am also giving something else a go. There is a woman - Inna that has set up "Comprehensible Russian". She seems to have a different approach. No or little grammar, and slow speaking 99% of the time in Russian. I hope that by using her as well, then my level of comprehension and speech will improve because using Duo is not all that helpful, especially with a drunk robot speaking or should I say mumbling. There are only two reasons that I have not canned this bl*^dy course 1) I hate giving up half way through anything and 2) there might, just might still be a few useful things ahead in the second half.
Many Russian verbs 'govern' (are used with) a specific case – помогать/помочь always takes the dative. Володя им помогал. Volodya was helping them. Я помогал своей сестре. I was helping my sister.
Good textbooks often give this information by including the question words in vocabulary lists; so you 'll see помогать/помочь (кому? чему?), or интересоваться (кем? чем? instrumental), for example. It's good practice to memorize this whenever you learn a new verb. :)
Take the following two sentences:
My sister is helping me make money.
My sister is helping me to make money.
In the first sentence the direct object will receive money with the help of the subject. In the second sentence the subject is giving help in order to receive money for themselves.