First, in English, it depends on what you are trying to say. There are times when a "the" would be entirely appropriate, and there are times when the zero article would be better. There is no "right way" and "wrong way" to structure a sentence like your example.
For instance, consider this:
Nothing warms my heart like seeing sisters happy.
That sentence is grammatical. It is saying that I like it when I see sisters rejoicing together. Note: I'm not talking about my sisters, I'm not talking about your sisters, I'm not talking about my mother and her sisters. That sentence is talking about sisters in general. It could be three sisters in Cameroon, it could be two sisters in northern Canada, it could be five sisters at a family reunion in Warsaw, Poland. Whenever I see sisters being happy together, it warms my heart.
Now for another example:
I cannot express my feelings of joy when I see the sisters happy.
This is also grammatical; however, by including the word the, it's obvious that I'm talking about a particular set of sisters. Suppose there is a book about two sisters who were orphaned and separated at birth, and then were reunited when they were much older. The narrator of the story might use the sentence I've written above, and we know the narrator isn't just talking about sisterly happiness in general, but a specific instance of it between the two sisters in the book.
Now for something closer to what you were trying to say in your examples:
A brother cannot express his enormous feelings of joy when he sees his sisters being happy.
Unless the brother is talking about sisters in general, we need some sort of determiner to express which set of sisters he is talking about. Now, if he's talking about his own siblings, his would be a much better choice than the. But other determiners could be used, depending on the context. For example:
When I was growing up, the family who lived across the street from us was very poor. But they always had joy, especially around the holidays. I would have never expected that my brother would stay friends with that family for decades and decades, but he still tries to visit them at least once a year. My brother can hardly express his joy when he sees those sisters being happy.
- Do I need a determiner before the word sisters? (Answer: No, not if you are talking about sisters in general.)
- Should the determiner be the word the? (Answer: Only if you are referring to a specific set of sisters, and the listener/reader knows who that is. Otherwise, it's probably best to use something more descriptive, like my, his, our, these, etc.