Systems and methods for stimulating wells include a frac window system positioned in a first wellbore adjacent a secondary wellbore extending from the first wellbore so that an opening in the frac window system aligns with a window in the first wellbore casing. The frac window system includes an elongated tubular with annular seals along the outer surface above and below the opening in the elongated tubular, and further includes an orientation device carried within the tubular. A main bore isolation sleeve is positioned within the frac window system to seal the opening, isolating the secondary wellbore from pressurized fluid directed farther down the first wellbore. A whipstock seats on the orientation device so that a surface of the whipstock is aligned with the secondary wellbore window of the first wellbore casing. The whipstock guides a straddle stimulation tool into the secondary wellbore from the first wellbore.