That is wonderful that your DH is so supportive as it would be really hard otherwise. Don't worry about your SIL. You will find it is a common reaction for people to be somehow offended by vegetarianism but after a while they realise that 1. it isn't a big deal and 2. you aren't criticising their choice to eat meat.

Good work on the recipes and the dietician. There are a huge number of online sites with recipes and you will soon find a way to substitute all of your favourite foods.