Does my sister have an eating disorder?

I don't have an eating disorder but I'm posting here because lately I've been worried that maybe my younger sister does. She doesn't look anorexic necessarily, she's about 5'4 and underweight I'd guess. Whenever I go to her house for dinner everything she makes for dinner is super healthy and significantly smaller portions than what I eat. Today when I talked to her she was crying because she took her little guy to the doctor and the doctor was so concerned about the fact he's not putting on weight that he's referring them to a dietitian. My sister is furious about the fact that she's being forced to see a dietitian. I know she's not intentionally starving her kids as they do eat but since my sister feeds her kids the same meals she eats they probably aren't getting enough calories. Another thing I found to be strange was the other day she posted a picture of a pan of brownies on her social profile and said she ate that for a snack but then I thought that was strange since I had been at her house the night before when she made the brownies and I was the one who ate most of those brownies, not her. My boyfriend also mentioned that when we went to a restaurant with her and her husband a few months ago she mostly just moved food around on her plate but didn't really eat it. I don't think she has anorexia or bulimia or if she is just so obsessed with healthy eating that it's become a type of eating disorder? How can I be supportive? I talked to her about it and said to just be cooperative and do what the dietitian tells her to do but she seems to be in denial and convinced that a diet of mainly vegetables is a healthy diet.

It sounds to me like your sister does have anorexia. too far underweight on a 5'4 in frame probably is below percentage of predicted ideal body weight, which technically meets the criteria for anorexia (anything below a certain percentage does). The fact that she's lying about what she ate and is just playing with her food, rather than eating it is also really worrisome. I think it's a really good idea that she's going to see a dietician, I only hope she listens to her and nips this in the bud. Once eating disorders take a hold of you, they have a way of holding on tight and being very difficult to get away from.
Even if she doesn't technically have anorexia or bulimia (and she likely does have anorexia), there's an eating disorder called orthorexia, in which the person becomes fixated on eating only "healthy" foods and absolutely nothing else. This may be playing a role too. Regardless, she would likely benefit from an eval with her medical doctor and an eval with a therapist.
As far as how to be best supportive, don't push her, she'll just push back, just firmly explain your concerns, tell her you'll be there to support her and maybe offer to go with her for moral support to any MD/psychologist appts. You could also maybe help by looking up those names for her or by finding a support group in the area so if she agrees to seek help, you already have a set of contacts for her.
As far as the kids go, that concerns me. I hope she realizes how big of a deal this is and makes a sincere effort to see the dietician and follow through with their recommendations. I'd stress your concerns for the kids and encourage her to do these things. She could very easily lose her children for neglect if she's not compliant with the recommendations.
I hope this helps some. Good luck. I hope she gets the help she needs.

I hope your sister does not have an ED, but the warning signs are certainly there. Have you ever broached the subject with her? Ultimately you cannot convince her to get help even if she does have one, but it might plant a seed for later. I am the oldest of four kids (well, we're all adults now) and each one of us has had eating disorder issues and what you have described definitely sounds suspicious, at the least.
I agree with not pushing her, but explaining your concerns and describing what you have noticed. It is definitely worrisome that her child is being affected, not only because of his health now, but because, if she continues to have a skewed perception of food and eating, it will have an impact on him growing up and how and what to eat (or not eat).
I hope you are able to get the answers you need. And I hope that if your sister needs help, she is willing to get it. Good luck.

it also sounds to me like she could have an eating disorder. i struggled with one myself but before i did had a friend who had anorexia. it was difficult for me to know how to support her because i was so worried about her. she was in denial about it to a degree which made it more difficult. the difference was we were both 14/15, so eventually her parents became involved which got her into treatment/receiving support.

i would be concerned about her children being affected by it since you mention they are potentially not getting enough nutrition themselves.

i like the idea that was mentioned about offering to go with her to appointments, etc.

the only issue will be if she flat out denies there is a problem and refuses to do anything about it. unfortunately, since she is an adult, unless she is a danger to herself out her children, she cannot be forced to deal with it and has to get to the point herself where she wants to change it.

is she a highly anxious, stressed out, or depressed person? sometimes that can also play a role in a person's appetite being decreased, etc. i guess it would all depend on other symptoms she has, if she fears weight gain, if her life revolves around food/avoidance of food, etc. too. symptoms can vary from person to person, and she might not yet have a full blown eating disorder but more disordered eating...which still warrants support and evaluation from professionals too.

all i could suggest is just not becoming upset with her since she might react defensively and shut you out. that is common when people aren't ready to talk about it. just let her know you care about her and are willing to just listen or be there for her if she wants you to be.