My Mother-in-Law and sister’s wedding

Not in my experience - I've never known the in-laws of a BIL or SIL be invited to a wedding, unless they happened to be friends anyway - they're not family

I wonder if at least part of this comes down to proximity. Most of my family and Dh's family live in separate counties to us and each other, so the idea of combining them in social situations doesn't really occur to us.

OP, is your wife of a culture where there are expectations with regards to guest lists and who pays for what? Everyone here seems to be applying contemporary British expectations here but so many weddings in this country are celebrated with large guest lists including extended family, especially when multiple cultures are represented.

I would invite my sister’s in-laws, yes. I’ve spent Christmas with them, they absolutely dote on my sister (more so than our own parents), and they’re lovely people who would be great value at a wedding.

At Christmas, your children's birthdays etc don't you have your parents and your in laws round? So they would know each other? And your MIL would attend as a guest of your parents? Are your parents contributing to the wedding?
To me that would be usual, unless it's a very very small wedding?
Is she entitled to an invite? No
Is it strange she wasn't invited? Absolutely

I just wonder if there is a bit of resentment from your sister towards your in laws. You have said your MIL has contributed to your house, your wedding, your career, and your life would probably be quite different if she wasn't so generous? Are your sisters new in laws as generous?

In my family it would be considered deeply weird to invite the in-laws in this situation. What do they have to do with your sister 😳

I was invited to my brother's sister in law's weddings (two of them) but that was a complete surprise. I thought it was lovely, but I would never have expected it. But then, I am of the mind that you should never expect an invitation to someone's wedding no matter how close you are to them. It is their day and they will have their own reason for who they invite. I am always very happy when I do get an invite though.

I do, however, think it is a huge faux pas to request to be invited. So your MiL and Wife need to revisit their etiquette books.

My MIL was invited to my sister's wedding but that's because they've known each other for years. They didn't invite the in-laws of the grooms siblings though. We didn't invite my brother in laws parents as we didn't really know them.

Did/would you really invite the parents of your brothers’ and sisters’ spouses to your wedding?

Does your sister know your MIL? Does she know her well? Are they close friends who spend time together?

I've met my siblings inlaws once in my life. At my siblings wedding. I'm not sure we exchanged more than a couple of polite words.

The idea that I should invite these complete strangers to my own wedding is bizarre.

It sounds to me like cultural differences might be at play. Your MIL paid for your wedding and likely had influence over the guest list.
If your sister is paying for the wedding herself, the loyalties of your parents are not relevant. If her/your parents are paying, I can imagine your MIL being upset.

I disagree as it depends on the relationship they have. If it's a distant relationship then fine but she seems to think they are family since the marriage. Obviously they are not.

People now get so carried away with the perfect wedding and don't care what relationships are fractured along the way, it's the wrong approach but hey everyone do that's they want

It would be normal etiquette for her to be invited, regardless of financial support.

Definitely not the norm but it had never occurred to me before now.

As for the OP, I think his MIL is wrong to expect an invite.

That's clearly not the norm. Seems OPs MIL thinks they are now family since they are.married but OPs side of the family thinks otherwise so it's just a case of everyone set expectations so people can act accordingly.

My mother and in-laws haven’t even met. After 9 years of marriage, this has only just occurred to me.

Why are you interested in women's views? It's your relationship with your MIL that's at risk, what do you think the right thing to do is?

For me I think when we get married we join another family and I would invite my brothers in laws unless there was some history or background, but obviously this is mumsnet where the "norm" is you invite who you want with no care or thought of how it impacts relationships.

My husbands parents came to my brothers wedding - would that be the same? My parents paid towards the wedding and invited them as their guests, but my brother knows and gets on with my in laws and appreciates how supportive they have been for us, so I think would have invited. Also, my daughter (their granddaughter) was a flower girl so it was lovely for them to see this.
Does your sister have to invite her? No, she doesn't have to invite anyone in particular, she doesn't even have to invite her own family
Would it have been nice is she had invited someone who had shown her sibling such generosity, and so they could see their granddaughter be a flowergirl. Absolutely.

Does your sister have a relationship with your MIL? It seems a bit weird if not.

I have a few siblings. We are all married. It would never have crossed our minds to invite each other's in-laws.

My mil does buy presents for all of my nieces and nephews. However, she does this out of kindness and expects nothing in return.

Stand firm. Your wife is wrong.

Oh, and if it is so that they can see DD as a flower girl, IF ONLY there were some way of sending them some kind of picture of what she looks like. If only someone could invent a machine that could capture images, and even sound, and transmit them to another machine a distance away! (I think this is my dragons' den moment!)

Is it a cultural thing?

So which of your sister's two close friends / close family should she disinvite so that your PILs can attend?
It was stretching it a bit that your parents were invited previously, but I'm sure they 'paid' for the invitation by giving a gift. The invitation does not have to be reciprocated.
It's incredibly rude to insist on invitations!

Tricky. I sort of agree with your MIL, and sort of agree with your sister.

Your MIL hasn't actually said anything though, which is the decent thing to do. Really it's your wife talking to you about this on her behalf (at her bidding?).

I think the only thing you can do is say something to your MIL along the lines of "I wish you could join us at [sister's] wedding, would have loved for you to see [daughter] in her flower girl dress. I don't know what my sister was thinking in terms of guests, wish I could say something to her but it's a bit tricky as I don't know the situation with her fiance and his parents. Such a shame, you've always been so hospitable to my family".

Basically, acknowledge the situation, don't take responsibility for it (because you can't) and don't apologize for your sister either (it's her wedding). It literally is just a regrettable situation.

And tell your wife to knock off putting pressure on you. Not cool, when you can't do anything about it.

Give MIL your sister’s number.
You need have no further involvement.
Simple.