Conflict is not argument. Two people shouting can be entirely without conflict if neither wants anything from the other; two people politely agreeing can be full of it if each is trying to get something the other will not give. The presence of raised voices is one of the least reliable indicators there is.
It comes in the familiar three registers — against another person, against a circumstance, against oneself — and the useful observation is that a strong scene usually has at least two of them running at once. The character arguing with their sister is also arguing with their own decision.
The reason conflict is non-negotiable is that it is what makes a scene readable as drama rather than as information. Take a scene where somebody explains something to somebody who wants to hear it, add the smallest reluctance on either side, and the same information becomes watchable.
What are the types of conflict in a story?
Conventionally: against another person, against circumstance or environment, and against oneself. The strongest scenes usually run two of them simultaneously.
Why does my scene feel flat even though the characters are arguing?
Arguing is not conflict. If neither character is trying to get something from the other that the other will not give, there is nothing at stake in the exchange and volume will not supply it.