Do you think I should say that my sister in law is putting on weight?
Are you going to say that to her face?
I intend to tell her in front of the whole family.
It is wrong to shame someone for weight gain in front of others. If you are concerned you should speak to her privately.
Saying that someone looks like a seal is not shaming them.
It's bad enough to say that she's gaining weight, but it's especially terrible to say that she looks like a seal. I think it would be best if you just kept your opinion to yourself, because I don't think saying something will help her.