How to deal with people who insist you're overweight?

Sounds like a good plan! I hope it goes well!

Turn it back on them.
Put on your best distressed look and tell them how worried you are about them since they keep bringing up the same subject every time even though every one else can clearly see how you have a perfectly healthy body and you've asked them to stop. Their obvious lost grasp of reality is worrying, clearly they have a form of body dysmorphia and also problems with their memory as they can't remember past conversations. Have they seen a therapist? And their doctor about the memory problems? Sudden changes in personality as saying such rude things to people about their appearance can also be a symptom of dementia.

Then do that every time.
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Oh wow this is AWESOME advice!

Tell them you will not discuss it anymore and if they are so worried, then they should go lose 5lbs themselves.

As long as you are happy with yourself don't sweat what they think. Tell them you are happy with where you are at and that you appreciate their concern but you are happy and that's all that matters to you.

Smile, nod, agree, thank them... Then carry on doing whatever you are already doing. The next time they bring up the topic... Smile, nod agree, thank them... They will get bored

I can just imagine how annoying this is... I have a similiar situation, but it's the opposite. When I was heavier I'd hear from my family (particularly my mother) how I need to lose some weight etc. and now that I've lost the weight all I hear is "OK. You can stop now, you look good now, don't go overboard... you're starting to look gaunt". I don't think I'll ever be considered "You're looking great, I can stop saying my opinion on you now". However, there have been times when I've had to say "I don't know why my weight and my body affects you so much! I am healthy and happy, time to leave me alone" and walk away.

In your situation, if I were you I would do as the others are suggesting and be really firm "____ I do love you, but seriously I cannot be around you anymore if you continuously talk about my weight and put me down. I don't know if you realise it, but whenever you say those kinds of things you make me feel _____ and all I am going to say is for the last time, I am happy, I am healthy - end of story. If this comes up again, I will have to walk out on you. Do you understand what I am saying and asking of you?" If they don't or they continue to go on about it, plain and simply walk out. No one should ever make you feel terrible about yourself and make you feel inferior. What kind of love is that!? Especially when you ARE healthy and happy!

Some people have a sort of distorted view of themselves, or lack confidence. The easiest way for them to gain self-confidence is by being better than someone. If you were the "fat one" this family member compared themselves to, and now you are not, your success kicked a leg out from under them. They are now convincing themselves (and you) that you are still in some way inferior, which by default, makes them superior.

You like how you look, and that is all that matters. This person is going to have to deal with their own demons, or find another "fat one" to hang out with. You can try having a heart to heart, and letting them know how they are making you feel, but I wouldn't bet on making a change. Those kinds of changes have to come from internal motives, usually.

Exactly this. It's not very nice at all. Some people don't like it when others around them try and change their life for the better as it makes them look at themselves. If this person compared themselves to you previously and thought they were superier they are more than likely not liking the positive changes you made.

It's hard but try not to let it get to you. Some people just can't stand other being happy and confortable in their own body xx

There is nothing you can say that is going to explain it to her. If there was, your explanations would already have worked. So give up explaining firstly, it's annoying you and it doesn't work. Plus, it keeps you on the back foot, you are the one having to explain yourself, she is the one asking questions. You need to realise that the person asking the questions has the power, the person explaining is not going to get power by explaining. Never. What you need to do is become the one asking questions. This will put you in the position of power, and it will deflect attention away from you. So as soon as you see her, you tell her that my gosh she looks thin, has she been ill? Is it healthy to be so thin. You've heard it can affect fertility, is she concerned about that? Does she find herself getting cold more often? Very thin people sometimes get a fuzzy growth of hair on their body, has she noticed herself becoming hairier? Where is she on the BMI chart, is she in the normal range? Basically you act like a concerned Mum, fussing over her health but really, she is the one being questioned, she is the one who has to explain herself. Everytime she tries to bring up your weight you say, I'm so glad I'm a healthy weight now, but honey I am so concerned about you, are you sure you're eating properly? Do you have an iron deficiency? Have you been tested for an overactive thyroid? Etc. Very soon, she will stop bringing up the weight conversation! Good luck!