I see that you are not being rude that you genuinely don't understand so it's with kindness that I beg you to please not say things like this to your DSis. Please read up on ARFID, it's really not as simple as well don't offer it, people who have this will literally end up in hospital rather than eat only what's offered if it isn't on their safe list. It's a very complex disorder and takes a long time and a lot of patience to understand and help expand their diet. Both my children have it to a degree and thankfully my DD has over the years gotten brave enough to try new foods and my DS doesn't have as restricted a range and is also slowly improving.
It's so so hard to parent a child with ARFID and it's really not well known, for your DSis and her DC sake I beg you again to research it and be there as support and to educate others as she will already be having a tough time without the ignorance and judgement you've (inadvertently i know) shown.
Take ALL pressure away from mealtimes and food in general, offer new foods one at a time (you can mix it up rather than only offer potatoes until they try it and then move on to pasta for example but don't offer potatoes and pasta as two new foods together) and know that you can offer the same new food item 15+ times (minimum unless you get lucky) before it's touched with a finger, then 15 times before it's sniffed, 15 times before it's licked, 15 times before a bite....the 15 is a VERY rough guide it could be less it could be way more but the important thing is, it's no big deal, it's offered and if refused no biggie just leave it sitting out for a while within reach and then try again at a different mealtime/snack time.
Massive praise for every each tiny step (touch, sniff, etc) but without making it a big deal (yeah I know contradiction right!) Don't offer a reward for trying or attach ANY sort of rewards regarding food, with a no biggie attitude, a LOT of patience, you can help someone with ARFID expand their safe list of food and in the meantime if they eat the same thing every day at least they are eating.
If you get lucky they will have at least one item from each food group on their list, if not look into what vitamins may need to be offered as a replacement whilst you work with them to know other foods are safe.
You can also help by eating a range of food yourself around DC, comment on it casually (remember no biggie) about the taste, texture, do you like it, not like it, what similar foods taste the same? A casual comment here and there, not every time, maybe once twice a day not meal might seem to be ignored but DC are sponges and they will absorb the information even if they don't show a response to it, you can say it to another person or directly to them "how's your potato DH? Mines really soft" "how does insert safe food item here taste DC?" "Oh really this food tastes similar" don't offer the similar food or maybe you'll like it comment just drop the info in a no biggie way and move the conversation onto something else.
Seeing their peers especially can help, so in nursery if all the children are eating chicken for example then DC will see oh well it must be safe as all 20/25 of my classmates ate it and weren't sick...it can improve with age.
Sorry I didn't mean for this to be an essay @Ashlll it's just slightly triggering for me as none of my family or friends had ever heard of it either and I got a lot of ignorant comments until I educated others on the condition. Please apologise to your DSis and she will understand it was ignorance in the good way not the bad way and she will appreciate your support as its bloody hard when all you want to do is beg your DC to eat