If you have told her that you are seriously concerned about her health and your daughter has brushed you off, there is sadly nothing you can do.
21 months seems a long time to be on anti-depressants for post natal depression, but again, if your daughter won't listen, you can't help. Have you tried mentioning your worries for your daughter to your son-in-law? He too may be worried about her.
With regards to his lifestyle, he seems either not to believe that his father's early death was related to overweight, or is ignoring the connection. If his wife can't change his attitude, you won't be able to either.
I am very much afraid you will need to bite your tongue and hope and pray that your daughter sorts herself out.