Thanks for the feedback.
This person is a relative, so I can't just refuse future invites.
And for those who wondered, I did ask about the lack of potato, and she replied, 'I put bread next to you.' So maybe I was reaching when I wondered if it was a snark about my weight...but then, I rarely eat bread, only if it's hot rolls or something really special. I can go months without a loaf of bread in my house. This was not hot rolls.
I think it would not have been rude to have said "Why would you do that?" Did anyone else offer to share their potato with you? I think if I had been another guest in this scenario, and the host did;t step up, I'd probably offer to split my potato with you and we could both make up the difference with bread, if we wanted.
I agree that whatever her motive, host was rude to treat you differently and rude not to do something to rectify the error (if it was an error) when it was flagged up.
Even though this is family, you would be fine to refuse future invitation. Alternatively, I do not think it would be inappropriate, in the context of a family relationship
, to speak privately to host and to say "I found it upsetting and embarrassing that you treated me differently to everyone else when I was there for [meal]. I don;t know whether it was an oversight or if you were trying to control what I should eat, but either way, I would prefer that you treat me the same as everyone else. If there is anything I can;t eat I will take responsibility for ensuring that I let people know, as need be"