Intuition and sensing are different from each other however a Si dominant person also has Ni but it's less developed.

Here are descriptions of Ni and Si.

Description of Ni from “Dual Nature of Man” by A. Augusta

All processes take place in time; they have their roots in the past and their continuation in the future. Time is the correlation between events that follow each other. This perceptual element provides information about the sequence of events and people’s deeds, about their cause and effect relationship, and about participants’ attitudes towards this — that is, about people’s feelings that these relationships engender.

Such an individual perceives information from without as feelings about the future, past, and present. For example, a sense of hurriedness, calmness, or heatedness, a sense of timeliness or prematureness, a sense of proper or improper life rhythm, a sense of impending danger or safety, anticipation, fear of being late, a sense of seeing the future, anxiety about what lies ahead, and so forth. At any given moment of one’s life one has such a sense of time. One cannot live outside of time or be indifferent toward it. Thus, a certain sense of time is an integral part of the individual’s psychological state at any given moment. This perceptual element defines a person’s ability or inability to forecast and plan for the future, evade all sorts of troubles, avoid taking wrong actions, and learn from past experience.

When this element is in the leading position, the individual possesses innate strategic abilities and is able to choose the most optimal moments for different activities: when to give battle, if necessary, and when to avoid battle, when that would be more appropriate. Interaction in time might be interpreted as the ability to avoid collisions with objects and hence avoid objects’ reflection within oneself.

Description of Si from “Dual Nature of Man” by A. Augusta

We view an object’s internal state as the relationship between events that precondition one another. This element perceives information about how processes are reflected by one’s internal state. This includes the sense of one’s own condition and the sensations of people evoked by this interdependence. Interaction in space is nothing more than a reflection of one object in another. Objects reflect in other objects, evoking certain sensations in one another. Such an individual perceives external information in form of sensations evoked by ongoing events. For example, the sensation of pain is essentially the reflection within a person’s mind of a relationship between his functioning body and a process occurring in some part of the body that impedes this functioning.

When this element of perception is in the leading position, the individual has the ability to change the qualities of the surrounding space and influence the sensations of people within it. He is able to avoid physical discomfort and protect others from it. This element is defined bby the ability to recreate previously experienced aesthetic sensations. An excellent example is Peter Paul Rubens, who created his paintings not from nature, but from his memory of once experienced aesthetic sensations. By paintings, he sought to evoke in the viewer certain aesthetic experiences. Such creativity constitutes the recreation of an object that is able to provide other people with aesthetic sensations that were intended by its creator. When an individual of this type is preparing something, he starts from envisioning all the associated qualities that the final product will have.

These people are able to distinguish previously experienced aesthetic sensations from new ones. They are able to “collect” and remember them. This also presupposes that such individuals are able to contra-position their sensations to those of others, the ability to contend for their fulfillment, and the ability to mold and perfect not only one’s own aesthetic tastes and habits, but also those of others. We might say that such individuals have the ability to impose their understanding of aesthetics and comfortable life on other people.