Learn Russian: Моя сестра не понимает этот фильм.

The dictionary form is понимать (to understand). Since it ends in -ать, it is a first conjugation verb. To say 'she understands' (matching сестра), we drop the -ть and add -ет, giving us понимает.

Where is the word for 'does' in 'does not understand'?

Russian does not use helping verbs like 'do' or 'does' to make a negative sentence. You simply put the negative word не directly before the verb. So, 'does not understand' is just не понимает.

'This film' is the object of the sentence. Shouldn't этот фильм change its ending?

You are right that direct objects usually take the accusative case (the form used for objects). However, for masculine nouns that are non-living (inanimate) like фильм, the accusative form looks exactly the same as the dictionary form. The word этот (this) also stays the same to match it.

Why use Моя instead of Мой for 'my'?

In Russian, possessive words like 'my' have to match the gender of the noun they describe. Since сестра (sister) is a feminine word, you must use the feminine form Моя. If it were a masculine word, like brother, you would use Мой.