My sister's son is 5 years old, but it's like he has a slurred speech; we think he has a speech problem; he doesn't pronounce the words properly; there is other children in the family that is younger then him that speak and his weight is not right ?

Speech therapy: I would have him evaluated by a speech therapist to see what if any work he needs. In the US, this is part of the pre-school, school system.

Any competent pediatrician should be able to assess the child & either make a diagnosis or refer him to someone who can. Please repost & let us know what his pediatrician says; or, if he has not seen a pediatrician at this relatively late date of 5 years old, please state why not.