Many parents become lax when sleep training their second, third, and fourth tots because they're afraid of waking their older siblings. According to Dr. Karp, there's no need to worry. If you're using the swaddling and white noise methods from birth, the baby shouldn't be making much noise.
He adds, "if you have a 5-month-old that isn't sleeping, start using the white noise. Give it four to five nights and see if that works. Probably 70 percent of the time you can finesse your way out of a sleep problem just by adding white noise. But if you're determined you're going to let your baby cry it out, with your older child, put white noise in his room."