What a great thread! I agree with pps who said that aiming to "get your old body back" is unhealthy, or at least off the mark. For myself and all my mom friends that I've talked to about this IRL, our bodies change after childbirth. In ways that can be hard to adjust too but also in wonderfully surprising ways. In my case, I've always been slim but I had a very round "bubble butt". My husband loved it and having curves back there is desirable to many people but I always hated it, mainly because it was so hard to fit pants. Shopping was embarrassing and frustrating. After having my first child, I was heavier than ever before (I gained 60-70 lbs despite lots of vomiting!) but I was a lot happier with my proportions. While other areas grew, my butt totally slimmed down! I had to go up a few sizes but clothes fit better and more comfortably. I knew I wanted to get pregnant again soon, so I chose not to focus on the positive (for me it was my new butt, maybe your sister has an area she likes better now too?) and not make any special effort to lose the weight, knowing I'd probably just put it all back on as soon as I got pregnant (aka the carb train) again. Some weight came off on it's own, and the rest I just ignored for the time being.
After DS2 was born, I still didn't worry about the extra weight again for a while. I saw the same weight gain happen to almost everyone around me and it was obvious that very few women go back to their "old body." I told myself that it's a rite of passage. My husband didn't seem to mind and again I focused on the positive - my huge nursing boobs balanced out my figure (I'm usually an A cup so going up to a C-D definitely helped with the bigger hips/waist). I knew I'd be more comfortable in my skin if I lost 15-20 lbs but I didn't feel any urgency about it. With two boys less than 2 years apart and renovating a big house, I had enough other things to be stressed out about.
Learning how to be a good, happy parent those first few years at home with two kids was harder than I expected but gradually I started to feel more like the mom I wanted to be and less chaotic from day to day. I was approaching 40 and starting to think about trying for
#3 . I felt though that I had to prove to myself that I could be in control of my personal independent life before adding the chaos that a third child might bring, so that I could have confidence that I could address instead of resent the sacrifices another child might bring. I knew it would be harder as I got older. So before getting preg again, I set some goals such as helping DS2 make progress with his separation anxiety, carving out time to read novels again, planting a new garden... and losing 25 lbs.
I'm pleased to say that I did it. It took about a year to get back to my pre-DS1 weight. I never went to the gym or followed a strict diet. I simply moved more - went up and down the stairs at home more quickly and more often, carried heavier loads of groceries, ran after the kids more, took them on more rigorous hikes (easier as they get older), hauled rocks in the garden, left the stroller behind and piggy backed instead, that sort of thing. For eating, I just did my best to cut down on sugar and wheat. I tried to have an apple instead of crackers for snacks, etc. More importantly I think, I worked hard at consciously parenting the way I wanted to and enjoying every day with my boys and my friends - this gave me enormous confidence and made losing weight feel effortless. For me, losing weight was part of a much bigger picture. To try to address it in isolation would have been futile.
I was proud of myself but I also noticed that I didn't look as good at that lower weight as I had before kids. My proportions had changed so much that I needed a few more pounds than before to fill out my chest and face, etc, to look younger and healthier. This is a great thing! I wouldn't want my pre-children body back for anything now.
Now I am pregnant again and packing it on once again - 10 lbs a month so far! But I'm okay with it because I know that this is how my body works and with lots of time and patience, it will come off...if I want it to. I have been down this road enough times to know that my body also looks just as beautiful, but different, at this heavier weight.
One last random thing that helped me a long the way - since having kids I've spent more time at beaches, water parks and pools than ever before in my life. Gradually chilling out and feeling okay about being in a bathing suit (it's about having a good time with the kids after all) and also seeing so many different bodies without clothes has been a great learning experience - nobody is perfect and everyone is beautiful in their own way. We're all just people after all. I love seeing baby bellies, misshapen tattoos, mysterious scars, all of it! We are all so different and ever changing on the outside.