CotswoldCountryMummy, as this is Relationships, I will really make an effort not to be (too) judgmental of your other thread on AIBU and give you some advice.
First of all, all of us have witnessed your metamorphosis from "lovely model DIL who stays with her PILs, cooks, helps in the farm and has never killed a fly" on your OP, to the spitting, screaming, wild-eyed monster who describes your SIL as "this rude, hurtful insensitive little cow" and says things like "its none of your effing business how long we've been married you sad cow" to other posters after they dared disagree with you. I am afraid we have t take you on your word, and assume that this is the way you react in real life when somebody disagrees with you. If this is true, then I wholeheartedly pity your SIL.
Your MIL is not your mother. She is your DH's and SIL's mother, and if anyone has the right to tell her off for speaking to her own mother in any particular way, that would be your DH, not you. If my SIL ever dared to tel ME off for talking to my mother in any way, I would let her know immediately and in not a very nice way what she can do with her opinions, and in which part of her anatomy she could shove them.
No one has the right of giving someone else's child an animal as a pet. NO ONE. It does not matter where that pet is kept, or who takes care of it. Owning a pet is a big responsibility, and only a child's parents can decide about that. I would be very resentful if my mother gave my (hypothetical) DD a pony. Actually, I would ask her to give the pony back, I would just not accept that "present".
According to your own depiction, your SIL is a miserable bugger. But the problem here is, you are giving us the depiction of somebody you personally detest. Your accuracy about that person's character is, therefore, deeply flawed, and I would be tempted to believe that your SIL may actually be a much better person that you say she is. If I had to choose, based on your thread, which of you to spend a weekend with, your SIL would have all the winning tickets.
personally, I don't see the point of couples inviting "friends in order to celebrate their 10th anniversary". It is ridiculous and presumptuous. No one cares about your 10th anniversary but for you and your DH (and perhaps your children, although I very much doubt this last). I will give you a personal comparison: my BIL's brother once organised a very strange "42 birthday party" for him, and invited most friends and family. Many people decided to skip it... how exactly is a 42nd birthday such an event? but wait, it gets worse... once all the guests were there, he shushes everyone up, and gives them THE BIG ANNOUNCEMENT: that the "real" reason for that party was.... him and his DW were going to have their vows renewed!!!
..erm... They have been married for 7 years...
He was probably expecting "ooohs" and "aaaawws" and congratulatory comments... in fact he got blank stares and silence. No one gave a shit. It was twee, it was embarrassing. My BIL was laughing so hard while he was telling us about this that he cried.
I believe you should get off your high horse (or should I say "pony"?). You come up as arrogant, interfering, and frankly difficult to stand. I do pity your SIL, and wish we all had her input in this story. You sound like a person that always tries to show a smile and show you are oh-such-a-good-person... but that has an ugly side that shows up easily. You seem to be trying to win yout PILs... and a cynical side of me wonders whether you're making plans to keep your PIL's farm and inheritance for yourself and your DH. You are horrible and judgmental towards your SIL and her family.
Right, end of my encyclopedic post. have a nice day.