Medical students across the US often like to rag on Family Medicine as an "easy" specialty, but the NBME shelf exam for FM is anything but. The challenge lies in its breadth, covering mostly ambulatory/outpatient topics from internal medicine, OB/GYN, as well as pediatrics - i.e. all of the types of patients a family physician might be expected to see. For this reason, it's recommended that you take your FM clerkship as late as possible, as the knowledge gained from past rotations will undoubtedly come in handy for family medicine.
AAFP Board Review Questions
I personally found this question bank to be excellent in terms of both breadth and depth. Best of all, it's free if you sign up for an AAFP account as a medical student. Currently, there's a total 1360 questions (split into 10-question blocks). Explanations are provided for every question. Although the difficulty is more resident-level, it will undoubtedly prepare you well for a medical student-level shelf exam.
Although UWorld doesn't have a dedicated section for Family Medicine questions, I found it helpful to do combined sets with IM, OB/GYN, and pediatrics questions. However, because there's ~1200 IM questions, many of which don't apply to an outpatient setting, I would prioritize UWorld only if you've already done most of the questions. Redo your incorrects and focus your time on a more FM-focused question bank like the AAFP board review questions; otherwise, try to focus on ambulatory/outpatient questions, though admittedly the UWorld interface doesn't make it easy to filter on this.
If you are looking for a more systematic, linear, book-based study approach, the ambulatory chapter of Step-Up to Medicine is a solid supplement to whatever question bank you use. It's about 40 pages long and covers presentation, diagnosis, and management of common outpatient afflictions such as hypertension, diabetes, pneumonia, etc.
An honorary mention goes out to Case Files Family Medicine. It's a solid volume, particularly if you have your family medicine rotation early in third year, but as the year goes on it's basically a rehashing of Case Files for IM, OB/GYN, and Peds. If you already have a decent amount of clinical knowledge and exposure under your belt, your time would probably be better spent doing questions.