Worried about my mum and sister

Unfortunately if they aren't interested in losing weight then there is nothing you can say or do. They have to want to lose weight for themselves. You can tell them about mfp, but until they decide that they want to do something to improve their health then nothing will happen.

The only thing you can do is lead by example.
If they see you lose weight, they may start to consider it. But don't say anything unless they ask....

Hey Laura, It's so loving of you to want the best for your mom and sister and to want to be gentle and considerate with your approach. It's so incredibly tricky.

Here's something I've noticed in my life as a teacher, writer, and just observer of life in general:

You can't actually tell a person anything unless they already know it or they have asked a question and you're providing an answer they sort of expect. They won't hear you. The trick, when you see something someone else doesn't, is to look for opportunities and ways to make them ask themselves a question that they can answer themselves. You don't say, "turn on the light so you can read better," you wait until they start rubbing their eyes and say, casually, "it's kind of dark in here." They'll find the light switch.

It's a slow process. Don't expect to have some magic words that you say and then your sister will be like, "oh, hey, I'm going to start logging my food, controlling my portions, and getting more exercise." It'll be more like she says something about how her weight is a problem in some small or joking way and you reply with a perfectly timed, perfectly tuned "really?" and then she says something else that sounds like the same habits are going to continue, but then weeks later you find out she started leaving some food on her plate or walking to the mailbox instead of driving.

Be a good example. Don't harp on it, but don't hide the fact that your management of your weight takes some attention, and is worth the effort.

Don't expect too much of yourself. You can't change them. They get to make their own choices and have the right to be as big as they want to. That's not your call. You can carefully, lovingly, at their pace, invite them to consider the ramifications of the choices they're making. Chances are, they already have, though. All you can do is keep listening and opening the door of curiosity about what it would be like to eat a little less, exercise a little more, and feel a little fitter.

Good luck! You're an awesome sister and daughter!

Every one is right. You can't make them do it. It really sucks to see someone you love having issues that you feel like you might know how to fix.

Maybe try and get them to do something with you that could push them a little....like a cooking class on how to make something healthy-ish like sushi? I think when you are 470lbs there are reasons besides hunger and love of food that you eat.

The trick is knowing what that is and finding something to replace it that brings you equal, instant happiness. Why do you think they continue to eat enough to weigh that much? Boredom, stress, life issues. Maybe you could try to engage them in activities that they'd like enough to gently help. It's kind of a round about way, but I've got nothing else.

OP, you can't "make" them do what you want immediately, but if you have the necessarily skills, which could be a tall order for many, anything is possible.

Find out their interests, fears, motivation and start from there. My brother and I have been talking about fitness and weight control with our mom for years. Plus we start to see people around us falling to illnesses. I'm toying with the idea that I'll gift my mom with gifts or money if she loses weight and takes better care of herself. It's in everyone's interests, anyway. It is making a dent.