To think I am not going to whither away?

Lifelong very strict vegetarian /vegan for several decades person here. I have not withered away, I am healthy, very active and am rarely ill. Mid 60s.

For an unrelated issue I had to have several tests and scans recently. Bone density is far better than average for my age, heart etc all fine.

Your BIL seems very uneducated about diets and nutrition but I suspect it is easier to ignore him than to try to change his mind.

Putting aside the bunkum you've written, people across the globe eat plant based diets, not only middle class western women.
Modern vegan food is much less expensive than meat, but a bag of lentils, one of brown rice and another of spinach will feed a family for several days, offering adequate nutrition and costing a fraction of any animal based alternative.

Potential Health Risks for Vegans:

• Bone Health: Studies have indicated a higher risk of total, hip, and leg fractures, partly due to lower average BMI and lower calcium intake.

• Mental Health: Some evidence suggests higher rates of anxiety and depression.

• Brain Function: Potential for cognitive impairment (brain fog) due to deficiencies like choline

• Muscle Wasting: Potential due to lower overall protein or calorie intake. National Institutes of Health (.gov)

Meat isn't necessary for protein and B12 vitamins provided you have the means and the knowledge to buy the required replacement foods to ensure you do not become deficient in B12 vitamins, iron and long-chain omega-3 fatty acids.

Without knowledge and money it's incredibly difficult to follow a "healthy" vegan diet which is one of the reasons it is predominantly followed by middle-class women in the West. It is however, incredibly easy to be vegan and very unhealthy if you do not have the requisite knowledge and available funds to buy vegan foods that are often quite expensive - nuts in particular cost more per kilo than meat - I know which one I prefer.

Veganism is a quasi-religion in that it shares characteristics with faith, such as a moral framework, strict dietary codes, and its cult-like members are always proselytizing.

So, in an average week you eat
1 portion of meat
2 portions of fish
a few eggs
some dairy
several portions of lentils.

To me this sounds like not enough protein. A woman needs a minimum of 0.8g protein per kilogram of body weight. I would suggest that you track your protein intake for a week or two to get a clearer idea of whether you are consuming enough. (There are lists of protein content of different foods online.)

Meat may not be a necessary component of a healthy diet for middle-class women with the education and means to buy the required foods to ensure that they do not suffer from malnutrition, particularly B12 vitamins.
Most people on the planet are not in that very privileged position.
Virtue signalling is an indulgence of a privileged minority.

In terms of protein, a simple rule of thumb is that you should be having the number you weigh in kg (say 70) in grams of protein per day minimum, up to about double (in this example 140g).

This is much easier to do if you eat meat and fish, and not impossible to do if you’re vegan/vegetarian, but it takes a lot more thought.

Muscle and bone density in middle aged women starts to rapidly decrease and doesn’t stop/reverse unless you actively do things which will build muscle and bone density (such as training with weights and eating a balanced diet with protein).

Judging by your meal examples OP, you have a fantastic diet and I’d love to eat at your house 😂 but I obviously dont know the exact measurements of each thing you’re eating in a day to know what level of protein you’re in taking.

Meat is the primary reason humans are at the top of the food chain.
Without meat the human brain would never have evolved the way it did.

So what?

These people are very much the exception to the rule. It's really unusual to be able to eat a vegan diet and grow/retain strength while powerlifting.
There are many moral/ethical benefits to being vegan, nutritionally it's still a significant compromise. Though the OP eating fish and dairy gives her a lot more leeway.

I would think that's because they aren't getting enough to eat.
Meat isn't a necessary component of a healthy diet.

Tell him to google vegan power lifters.

For example:

I eat meat maybe once a week, I just don't really like any meat apart from bacon and very occasionally something with beef in it. I eat alot of dairy, nuts and the occasional egg.. I definitely haven't withered away yet 😅

“Fish” is not included in the natural meaning of the word “meat” in English. That is why you often hear references to “meat or fish” on menus and in cookbooks

Nobody says “I’m going to the fishmonger to buy some meat”.

And yes, before anyone jumps up to point this out, I am well aware that some types of fish are described as “meaty”, such as monkfish, but that is because their texture is reminiscent of actual land animal meat.

I dont eat any meat.

What a weird reply. The OP is concerned about meat intake for protein and b12, we know that meat isn’t necessary for those things. I’m not talking about beliefs, religious or otherwise. It’s science.

Correct

Or eat red meat from cows that live in fields and eat grass all year as most do here.

Veganism is a quasi-religion and like all religions should be taken with a pince of salt.

You don’t need red meat for b12. B12 comes from soil, which most (factory farmed) animals no longer graze on and do not get it naturally. So they inject the cows with b12 supplements. So you can just take the b12 supplement yourself. Meat isn’t needed by most people. There are vegan strongmen and athletes. Plant proteins are great. Baked beans on brown toast contains all 9 amino acids.

Millions of people don't eat meat because it is too expensive - that doesn't mean they are as healthy as people who do. Nearly 40% of children under the age of 5 in India are stunted due to malnutrition. If these children had red meat several times a week they would be a lot healthier.

It’s worth asking a dietician to assess your protein intake. You may just need a few supplements or something, or you may be perfectly fine!

Surely fish is meat, it's animal flesh.

There's no protein in butter....