Is it safe/unsafe for a baby (9 mo.) to sleep on a regular mattress?

my sister in law has never bought her baby a crib and he slept in his car seat for 4 months. it is safe for the baby to be sleeping in bed with her? he is only 9 months old :/
Update: (thanks Jack K... i'm sure she wouldn't stop even if me and my husband ourselves bought a crib.)
Update 2: let me set the record straight, i'm 22, NEVER had a child of my own, so there fore i don't know these thing. hence why i decided to ask the rude people on yahoo answers. my 3 sisters didn't put any of their babies in the bed with them, so i figured it wasn't good for them. one of my sister's didn't nurse either of... show more let me set the record straight, i'm 22, NEVER had a child of my own, so there fore i don't know these thing. hence why i decided to ask the rude people on yahoo answers. my 3 sisters didn't put any of their babies in the bed with them, so i figured it wasn't good for them. one of my sister's didn't nurse either of hers, so it wouldn't matter. i never said i was going to buy a crib take it to my sister in laws house, set it up, out the baby in it and tell her they have to make him sleep in it. no. i just thought it would be safer for him and his health if he slept on a mattress designed for what he is...a baby. now i know that its okay. so thank you all.
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