Mabel is Stan's great-niece. Mabel's sweet, accommodating disposition and love of aesthetics often clash with Stan's forceful methods and sloppiness, occasionally leading to self-improvement attempts that Stan nevertheless goes along with tolerantly (like curing his fear of heights or overcoming his inability to speak with then-crush Lazy Susan ). Stan can be protective of his niece and, as with Dipper, he will get jealous when she spends time with others instead of him or expresses admiration of Ford . Stan appears baffled by and unwilling to participate in Mabel's more flamboyant plans, sometimes refusing to acknowledge that she is up to something weird or opposing her worldview entirely. He seems more capable of understanding Mabel than Dipper, often joining her in teasing Dipper and sharing interests (like their love of Ducktective). Stan notably has nicknames for Mabel (calling her 'sweetie' and 'pumpkin') whereas he has only referred to Dipper as 'Dippy' in writing. This ease in showing emotion for Mabel could be due Stan not feeling the pressure of having to toughen her up for the world.
This doesn't make Mabel exempt from his schemes, as she is shown to be involved as often as Dipper is.