I'm going to tell my 20 year old cousin, who has gotten fat fast since we last met, that she should get her weight checked and lose some weight very bluntly.
That sounds harsh, are you going to be more polite about it?
No, I'm not going to do it in a more polite way
It is wrong to tell people they are fat if you are being rude. It might be kind to just not give other people unsolicited advice and mind your own business.
I'm not going to stop, her checking her body is the first step to get healthy. She is going to get a rude awakening.
I get that you are worried about your cousin's health, but you need to use a more gentle approach. If you speak to her in a cruel way she'll feel attacked and ignore your advice.