Room sharing can certainly make things trickier! My girls are 3 and 1 and have been sharing a room for about a month and a half now, so I totally get it! Exactly how old is your toddler and his brother?
If your toddler is 2 or older, introducing a toddler clock would be super helpful to give him that boundary around morning wake up time. Our Hatch Baby Rest has been a lifesaver, as my 1 year old wakes up a bit earlier than my 3 year old. My 3 year old used to wake up when her sister did, but would still stay in bed quietly until her clock changed colors. Now I think she’s immune to her sisters noises, so she generally sleeps right through!
Here’s a blog post about using a toddler clock: https://viagraces.com/2020/07/06/how-to-use-a-toddler-clock/