The phrase "sink in" is commonly used to describe a feeling of understanding or realization. However, there are several synonyms that can be used to convey a similar sentiment. One possible alternative is "dawn on", which means to gradually become clear or understood. Similarly, "click" or "register" can be used to describe the moment when something finally becomes clear. Another option is "seep in", which implies that the understanding is slowly penetrating one's consciousness. Ultimately, the choice of synonym will depend on the context and the intended tone of the message.