My sister co-sleeps with her 9month old son. He's never used a cot of any kind. She's visiting from overseas and is staying with my mother. Even though we got her a travel cot, she puts baby to sleep in her bed (normal adult mattress) in a room alone to nap. I'm so worried about him. Is there less risk now he's 9months and has slept like this since birth?

Sudden Unexpected Death in Infancy (SUDI) includes SIDS & Fatal Sleep Accidents up to the age of 12 months.
90% of these deaths occur in babies less than 6 months of age…... so babies of 9 months of age have a much lower risk of SUDI than babies of 3 months of age.
Once a baby is mobile & no longer sleeping in a cot, there is a risk of accidents (for those over 12 months), including falls from adult height bed, as they may have access to areas in the house that may cause harm - eg windows/blinds/stairs
Red Nose “Cot to Bed safety”  is helpful in identifying some of those risks that may also be relevant for a baby at 9 months of age who is no longer sleeping in a cot

Red Nose has the following information about Sharing a Sleep Surface with a Baby.
There are some situations of co-sleeping that are riskier than others at any age.
Red Nose promotes a risk minimization approach to co-sleeping.

Sharing a sleep surface with a baby can increase the risk of SUDI. A considerable proportion of SUDI occur on a shared sleeping surface.
Babies most at risk of SUDI when sharing a sleep surface are those less than 3 months postnatal age, babies who were born preterm or small for gestational age.
The risks are always much greater if parents smoke or are under the influence of alcohol or drugs (prescription or illegal) that cause sedation and impair their ability to respond to their baby.
There is a very high risk of infant death, including deaths attributed to fatal sleeping accidents, when a baby shares a sofa or couch with an adult during sleep.
https://rednose.org.au/article/is-it-safe-to-sleep-with-my-baby