In-laws still serve me food they know I don't like

Same! My grandmother is Italian.

She either cooked with olive oil or lard (which has a lower smoke point than olive oil)...
And everything was absolutely delicious ;)

You say how hard is it? Well actually it is quite tricky cooking specific dishes for one individual (and additional expense). So whilst I do think they should accommodate you, I do think you are appreciative of the effort people go to when the do cook separate dishes to accommodate dietary requests.

My ex was vegetarian, and there was no way the family were going to give up a Sunday roast forever- which meant cooking separate potatoes, Yorkshire puddings and a main dish. It was a significant amount of effort. I hope you do recognise that for the people who do try to accommodate you.

Well it’s time for you to start getting over sensitive and taking it personally…. They are being incredibly rude. You are their DiL a vegetarian and yet they continue to serve you meat products.
Stop allowing them to control you.
Is your husband allowing this? After that many years mine would be having firm words….. like telling them we wouldn’t be dining with them if they carry on this nonsense.

As pp said:

Refuse to go there to eat anymore. OR bring your own food.

We use olive oil for everything. Spectacularly great roast potatoes 😁. You don't even need that much oil.

Is it ridiculous? Leave the children asleep and go get some food, then come back. I don’t see why you don’t do this actually, you should get a meal.

What do you mean they cause problems? Why do you feel you can't politely challenge this ridiculous situation?

Theyre certainly not sensitive to you..? It's very thick skinned of them to repeatedly serve you meat.

You have to accept this treatment for the rest of your life or follow one of the many suggestions on here.

I'd also consider a friendly phone call before you go. Ask if they remember your vegetarian, is the veg meat free.

Or get an uber eats delivered while you're there.

I really don't think some people understand about vegetarian food. I was a vegetarian for about 20 years and when we were on holiday once my DF ordered a cheeseburger for me as he thought it was vegetarian!!!!

Also, I've been offered a ham omelette in a cafe and the staff were quite surprised when I said it wasn't vegetarian (this was a long time ago - I'm sure there is much more awareness now)

I don’t have a single moral view on eating meat but eating it makes me feel sick. Literally. I haven’t liked it since I was a kid. Some people are allergic to it. It’s a thing. Would you really serve someone food they really don’t like? I like my guests on the whole and would prefer to feed them something they would enjoy.

I can't believe anyone is suggesting that the OP should bring her own food. I think when you're with extended family or friends, eating food together is just as much a social thing as a means of fuel. It seems pretty shoddy not to accommodate the people you supposedly care about, even if it's just something very basic. It shows you care. These people are sending a message and it ain't nice.

I am not the world's greatest cook, but I can manage to shovel some vegetarian options into the mix, not to mention something decent that's dairy and nut free for the poor allergy sufferer whose meal is invariably an afterthought everywhere he goes.

You really should stop going.

I wouldn't eat a prawn starter at MIL's as I can't stand prawns. They (in laws) thought it was hilarious that I was so averse.

Next time I went for food - soup to start, then surf and turf. Both H and I got up and walked out.

The rest is a very long story ....

They are being rude. If they’re picking up ready made stuff how easy would it be to pick up something veggie?

your husband should have a word with them.

Your husband should be standing up for you and saying to his parents that its not OK to keep serving you meat when you're vegetarian

Yes my PIL the same, they have certain anxieties which are known to us and we must accommodate. Like social anxiety - so they won’t leave the house for Christmas so when we (or another in law) host they won’t attend, we have to organise our day so we can visit, but not all at once. Hosting makes MIL anxious too, so we don’t have the option to spend it there. But things that make me feel anxious (feeding toddlers whole grapes whilst running around) are ridiculous and they will just do it to antagonise me and make a point.

You don't help yourself if the way you are picking fault with other people's suggestions is anything to go by. They are the ones being rude and they are doing so on purpose. It isn't the first time they've met you where they might have been absentminded and made a mistake. I would have returned the plate and said you'll plate up yourself without the peas and if they started to get annoyed pointed out that you being a vegetarian is not a new thing! If all you care about is keeping the peace then by all means let it continue.If you want to make a change then speak up for yourself and let your dh deal with the fall out. The fact that he doesn't speak up for you also speak volumes about the lack of respect he has for you.

I find that a bit of an odd, if not hostile, way to look at your guests. Yes you don’t ‘have’ to accommodate anyone, you could serve up a toy truck for dinner if that’s what you want to do, but any mature adult would accommodate a non meat eater if they were coming to dinner, wouldn’t they?

Are your in-laws the Royles? I remember Nana Royle saying “well, we’ve got wafer thin ham” when a vegetarian visited.

Seriously they’re twats. Perhaps take your own cheese sandwich?

You describe them as sensitive people.
However they know you don’t eat meat but repeatedly make meals to exclude you.
So take your own food or stop going. Don’t worry about their feeling when it’s clear they have zero regard for yours.

People are so utterly bizarre about food. By which I mean people who think that nobody is allowed to decide for themselves what they do or do not eat for any reason; that it's rude not eat whatever someone else plonks on a plate in front of them; and that not offering someone food that you know they won't eat is a perfectly reasonable thing to do.

Probably M&S ready made as OP says they do ready made roasties

I'd bring my own food - save them the bother.

You can roast with olive oil. The thing you shouldn't do is reheat previously heated olive oil eg use it in a deep fat fryer.

And that is why it will never change.

You really need to put a stop to this.
You have recognised that their moods are controlling so stop being controlled.
What does your DH do when they start?