It's not a problem for you to claim him (if qualified), and your sister to claim him next year. However, whether or not you can claim him depends on his age, income, whether or not you paid for over half of his support, and other factors. Please see this the following link about Rules for Claiming a Dependent to make sure he meets all the criteria.
Rules
for Claiming a Dependent
Who Can I Claim as a Dependent?
https://ttlc.intuit.com/replies/5187060
