To resent carrying all of the mental load of catering for DSD's dietary requirements

YANBU to resent your DH's refusal to take care of his dd, or even think about HOW to care for his dd.

Fish fingers
Fish pie
Salmon in different ways
Fish cakes
Quiche

DP needs to step up and take responsibility for planning to accommodate his daughter's special diet. If that means she gains weight, that's up to him and his ex to sort.

I'd make food that the rest of the family will eat and enjoy, regardless of what DSD is eating. She will have to accept that your kids can eat things she can't, if she's committed to following her special diet.

This does sound like a bit of a pain in the arse. If you are constantly trying to cater to everyone'spreferences simultaneously, you will go mad! To that extent, I have to say you are creating this mental burden for yourself. You need to simplify things as much as you can, and have a rule that when she's with you, everyone eats what she can eat. If your kids are fussy they can go without, or help themselves to a narrow range of healthy foods they like.

No child ever willingly starved itself to death. Few kids could have been fussier eaters than my eldest - until he was around 7or 8 I could count on the fingers of two hands the foods he would eat. Cream crackers, cooked chicken, cucumber and blueberries - I always had them in the house and if he didn't like what we were eating he could have those for his meal. As for DSD feeling sad when everyone else is eating cheese and she can't.... That's just life. She's going to spend most of her life around people who don't follow these religious rules, so she needs to get used to it.

But of course your husband should be pulling his weight. The thing is, if you want him to take on some of the mental load, you have to be prepared to let him do it his way, and that might not be your way.

If he was single I doubt he’d have her for this long at a time or cater to the diet his ex is insisting on. Like lots of step mums you’ve fallen into the trap of caring more than the parent does. It still won’t be enough, no one will thank you and you’ll feel very put upon. You’ve had loads of advice on the specifics on the food issues but take a step back in your head and consider what he’d do if you weren’t around to skivvy and think for him, for his child, for his ex and adjust accordingly.

If she is overweight WTF is she actually eating as part of this religious diet then?

This does sound extremely difficult OP. If your DH doesn't normally cook, he's not going to cope with preparing meals for DSD without a lot of input from you, so I wouldn't bother. I would try to find some kosher meals (e.g. roast chicken) that everyone will eat, and some kosher meals suitable for batch cooking and freezing so you can easily produce a separate meal for DSD that isn't a ready meal (but no harm in the occasional ready meal either). I would make DH eat the kosher meal too so DSD doesn't feel like the odd one out - that can be his contribution. Also re. the 6 hour rule, can you eat dinner a bit later to get round that, with a snack to tide the children over?

Everyone in this scenario is taking advantage of you.

Bloomin hell! That’s so tricky! I’d be saying to DH nope, she’ll eat what we eat or you can cook separate for her. Then leave him to his ready meals. If she’s overweight that’s a shame but you’re not going to solve it in one summer, nor shud u have to. The poor girl with a crap dad.

Can you buy vegan cheese for her?
So don't make everyone vegan but adjust bits for her?

And look at Cook ready meals. They aren't rubbish. They are really good quality.

It seems to be a very selective pattern . Time lapse between milk and meat but don't expect you to keep a kosher kitchen . I think it's unfair to put the burden of meal planning on you alone . DH needs to have some input too . Doesnt help that your own kids seem to be fussy as well . Bet you can't wait until everyone can cook for themselves.

This is absolutely on your DH, he should be sorting out her meals. If you need some ideas for meal planning, however, you could stick all the information into Chat GPT and ask it to create a meal plan that will work for everyone. I am aware it's the work of the devil but it is also pretty good at doing stuff like this 🫣
Edit: just seen PP has suggested the same!

Yanbu to be annoyed with your dh but I dont think thisnis impossible. Cook the meat separately if necessary. Revert to lots of meals with meat and veggiss and xarbs - grilled chicken, steamed veg, potatoes or fried fish, chips, salad. Make your own pizza.

Encourage your children to eat tomatoe based pasta sauces.

Will she eat quorn or similar?

He's a loser and you are being used by him.
Hand it over to him.
You are not married, this is not your child.
If it's important to him, he can sort it out.

Just don't mix meat and dairy and you're golden

How long is she with you for in total and how much holiday does your partner take while she is with you?

Already tell him next year you are taking your child ( that isn’t his) away for 2 weeks during that time, and he has to cater for all his kids during that time.

He is being allowed to opt out of parenting.

Will it upset DSD to just ignore her diet while she's with you? Or make her ill because her body is not used to digesting the food she doesn't usually have? If not, I would honestly just ignore it. If dad isn't on board with helping, then it's just too much work.

Your very lazy DH is surely capable of spending ten minutes writing a command in ChatGpt or Claude to explain what is needed for a two-week meal plan, with shopping lists?! If he isn't, you have a bigger problem.

This - l could not put up with this shit, the answer would be l am unable to cater for her needs and therefore can't have her for so long.

This is why I would never get into a relationship with a guy who had kids, then have even more kids together

What a disaster

This Is a conversation you need to be having with DH, not Mumsnet. Your DSD is 7, you could potentially find yourself in this exact situation, every summer for the next decade. Sort it now before it becomes the norm. I think it could work but needs a team approach, with you and DH sharing the load. Together sit down and work out a menu. Figure out which meals work for everyone and which will require a separate meal for dsd. Where possible ask DH batch cook and freeze a couple of options for when dsd needs a different meal. It’s fine to include a few days of jacket potatoes with tuna and salad, beans on toast, omelette.
I would also suggest DH takes this opportunity to educate his daughter, that, as she follows a different religion, there will be times when she can’t have what others are having (I.e the cheese on meat based pasta dish), she’s going to need to learn this anyway. Liken it to a child with an allergy being unable to have foods containing their allergen when others are.
There is also an opportunity for you to try to expand your own children’s palate, particularly challenging the notion that all meals need to contain meat and, as we all have different tastes, sometimes it’s necessary to have foods that you enjoy less in order for someone else to have their favourite.

No; the child is likely of the same religion: and it would be completely disrespectful to feed the child food that isn’t permitted in her religion

If you’re in the UK then the quality of vegan meat substitutes is actually quite good. My meat-eating DD actually often prefers them. So get in a supply of vegan chicken less nuggets or burgers then try and make some days ‘dairy’ days and the next day a ‘meat’ day. Using vegan substitutes for any cross-overs. Eg Linda McCartney mince for a veggie chili or spaghetti bolognaise.

And Yes, it should be DH stepping up to take the mental load.

You can insist he takes responsibility for it and cooks for her, you can't insist he doesn't give her a ready meal every night if that's the way he chooses to handle it.

YANBU to find his lack of concern for his DD's health startling poor parenting and deeply unattractive.

I would order a load of kids' meals from Cook (that she will eat) and give her that on Pie Day.

If it's a pasta dish I would boil all the pasta together then pop DSD's portion in a small separate pan and add tomato sauce. Even lasagne you can put her portion in a small pie dish.

Some frozen margarita pizzas might be a good idea too.

You could keep lunch easy like a PB/hummus sandwich for DSD

My kids dont like cheese or spice so I often make half of dinner then pop a portion into a separate pan whilst I add all the lovely chilli to mine!