@PenisPortions I have learned two things since I discovered Mumsnet:
One is, that it is (virtually) impossible to start a thread with one aim in mind and to get all of the other Mumsnetters to stick to the OP's one aim - especially in the AIBU, RELATIONSHIPS, CHAT, and IN THE DOG HOUSE topics.
The Second is that an Original/Opening Poster, will frequently enhance their story from the beginning of their first post, in an attempt to get any readers of their thread to agree with them asap - I believe that this is a common (and sometimes an unconscious) tendency amongst us humans, although I have seen some dogs use this tactic too!
The biggest problem that I have with the second tendency I spoke of above, is when the OP of a new thread has no compunction in continuing to be ingenious, in an ever more anxious attempt to get everyone replying to her to agree with her. Unfortunately, the person who is most negatively affected by the continued attempt at trying to get everyone else to agree with them, is usually the OP themselves. I have a picture in my head of Rumpelstiltskin repeatedly stamping the floor, until he makes himself ...
(the story has various different endings for Rumpelstiltskin, but they nearly all end with him not being very happy).
Anyway @PenisPortions back to you...
I am much more of an equal age to your MiL than I am to you, and at least in this case, my mindset is probably very similar to your MiL's too. I am going to 'speak' here as if I am your MiL and what she is thinking about when they arrive at your house:
"Oh no, we have been invited to have dinner with 'son's name' and 'dil's name' which we have gratefully accepted - especially as we were told it would be steak, which is my absolutely favourite meat, and as we are pensioners we cannot afford to have steak at home anymore. But my DDiL has just shown us the two large steaks - her description, not mine - that we will be sharing beyween us, and unfortunately I just can't bring myself to take my half of one of those steaks, as at least in my house that would leave each person with quite a meagre portion of steak each.
So what shall I do, I did hear a very quiet reference to lamb chops between my DS and my DDiL, but after a quick ssh, the lamb chops were not spoken of again (believe it or not, lamb and pork are my joint - no pun intended - second place choices for my favourite meats). I think that the best I can do is make up some excuse for not wanting to eat meat at all, at the moment, as the rest of them will then be able to have half a steak each, plus a third of the fourth half each.
I don't believe it! My husband has said that he is not very hungry, so he only wants half of one of the steaks (I know my DH very well, so I am 99.99% certain that he has said that, in order for the other two to have 3/4 of the two steaks each). I am very pleased that my DH said that, as I know that if he was given the chance to polish off a 12 oz T bone steak, he would jump at the chance, but with his kind offer he will only end up with an approximate 5oz steak.
Unfortunately, it has now become apparent that my DS is a bit of a greedy piglet, and a little bit selfish (I know, but I am his mum, and he was my PNB!), as he has just tried to fob DDiL off with half a steak, while he keeps the whole one to himself. I will be having a quiet word with him later..."
I do think that you OP - and your DH - misjudged the situation earlier, whether through ignorance or disingenuity I can't say (but I obviously have my own thoughts about that). I still firmly believe @PenisPortions that you should have either bought another two steaks, or you should have hidden the steaks you already had in the back of the fridge, and you should have cooked the lamb chops instead (with lashings of mint sauce!)😁😋
NB: I have actually learned a lot more than just two things since I have been a reader of Mumsnet 😉