To say she needs to continue with this?

[DilemmiaTime]

Adult sister is staying with us. She has 2 autoimmune conditions and is on a chemotherapy drug to control it.
She had a nanny role but had to quit due to intolerable conditions. She got a job working as supply in a nursery but is consistently ill. Long and brutal colds and coughs, 6 already this year. She never had so much as a sniffle when nannying. Caught covid in March and had that for a month and now 5 or 6 heavy colds afterwards.
Aibu to say she should try and carry on with the nursery supply work however as things are bound to improve over time? She can pick when she works so doesn't do much as it is and don't want her unemployed for a long period of time if she quits this. It's not about me, just want her to stay in some form of employment even if not doing much.

As an adult surely it is her decision?

I would say its not your business unless she is asking you to support her financially...actually even then its not your business, you just say yes or no.

If her health is affected by autoimmune conditions it might not get any better.
Depending on the conditions, how her body reacts to treatments and if the AIs cause any knock on conditions—- her health could actually get worse.

She needs to make her own decisions based on what is best for her health.

if a nursery role is causing her to pick up a lot of infections, why not look for another Nanny role?
She is an adult, her choice - or is your interest that she is living with you and needs to contribute financially?

It's not your place to tell her she needs to continue with anything, if she's an adult she can make her own mind up.
Are you worried about her being unable to contribute to your household if she's unemployed?