Sister-in-Law didn’t invite my mother in

The SIL didn't do anything wrong. She told OP's DM, in advance, that she wouldn't be available on that date/time. We now know this is because she had her family around. This could have been a light-hearted get-together with family, in which case inviting MIL in to join them could have been something nice to do in the moment. Equally, it could have been a family meeting to discuss a personal, maybe upsetting situation within SIL's family and none of them were feeling sociable at that time to meet someone new (and it would have disrupted their important conversation). The point is, none of us knows the reason SIL didn't invite MIL in, but I'm happy to accept that she had a good reason not to, and her MIL should accept that too without getting upset and potentially causing harm to their previously friendly relationship.

Mum needs to stop going out if her way to drop freebies off ... give them to the charity shop, might be more appreciative

It all sounds a tad insane, wherein your Mother shows up a few times a month with freebies, or whatever.
Your SIL doesn't have to explain herself when it comes to entertaining anyone in your own home and if your madre chose to ring the doorbell, then that's on her.
Yes, she could have invited her MIL in, bit maybe she has these pesky things known as boundaries?!

Seems pretty rude to me. I imagine she knocked as she interpreted unavailable to mean sil was out. Given that their was clearly someone home then knocking is pretty normal imo.
If SIL had said, oh sorry, can't today as I have my family round, could you make it another day or drop it in the porch' then that would have been better. But tbh it would have been far more normal to invite her in and Introduce her, 5 mins chat and then an 'lovely to meet you, I must be off now' from your mum and all social niceties would have been met.
Telling someone to leave something they are bringing you for free on the porch and then bugger off so not to bother you is pretty cheeky. Assuming they want whatever this free stuff is she is dropping off?

The weirdest thing is the MIL being this upset about... nothing really. It's a bit of minor awkwardness at worst. Surely it's possible to think of umpteen reasons SIL didn't want to add another guest to her family gathering (who hadn't met them before). Difficult family members, chaotic children, someone having an argument, about to serve food, house a tip etc etc. Why is MIL taking such umbrage and going crying to OP about it? That is a little dramatic - taking it so personally that you weren't invited in briefly to poke your nose into a gathering you weren't invited to at all in the first place.

I think it’s a bit odd she didn’t invite her in but she does need her boundaries in place. Otherwise when does it stop friends round etc. Sometimesit’s ok to keep things separate.

I'm with the MiL. I can see why 'leave it in the porch' implies that SiL is away from home or busy with work, if that is normal for them. So when she was clearly home and with guests it just seems rude to drop without saying hello. I'd have knocked too, if only to say "Hi, I can see you've got guests, but I didn't want to drop and run without saying hello". And then I'd leave. But if SiL didn't want MiL to even say hello, she should have asked MiL to come on a different day. It makes it obvious that for some reason she isn't welcome with SiL's family, and that's just a bit hurtful.

I think SIL was a little rude tbh. I think given families come together, out of respect for her partner and your DM she should have asked if she would like to come in and say hello to the family. Id be upset if my sil did this to my mum tbh. However, I also don't think SIL maybe thought it through properly and maybe doesn't think she's done wrong. Also depends on culture, because in my culture that would be considered disrespectful.

Based on the relationship you describe, your mother is not being unreasonable to knock on the door. And it’s a bit weird that your SIL didn’t invite her in. But, they had an understating so perhaps for her own reasons she expected a porch delivery to be just that.

Has your DB or the rest of you his family ever met her family? Perhaps some reasonable reasons for the behaviour.

As their relationship is good then your mother could/ should ask what that was about. Gently who it was etc.

Well I think the SIL was rude to ask her to drop the freebies and go and MIL rude to knock when SIL had said she was busy. Really SIL should just have said she was busy and arrange another time and not mention dropping stuff off so they both at fault.

Maybe your Mum should stop with the freebies although it does sound like SIL normally does spend time with her. When does your brother see her?

The only socially inept person here is the MIL who decided to ignore another adults message that she was busy and tried to interrupt SILs plans.
I can only think your mother feels entitled to your SILs time or access to the house and due to her own poor behaviour had a rude awakening that actually that's not the case.

I think your SIL was very rude.
I cannot imagine doing this to my MIL ( even way before we were married)

I agree

She's not owed an explanation or apology, she decided to intrude and knock despite being told SIL was not available.

My in laws didn't meet my mum until my wedding day, and they only met her two more times after that!

Iv a daughter she’s 14yrs old and the friendliest,sweetest,carrying person ever.But been bullied since primary school.She’s tried so hard to go to school as she wants to work and learn.But the torture from kids is too much.She’s forced he self to go for me but broke down.she has been taken out of school by cams and thrive.Reason. Is due to her mental health as she has been through a much,with bullying and loseing two homes being b made homeless due to private landlords.We lost our pets and all our belongings twice plus her mates.So she’s att home ALLDAY with me😭she’s depressed.I need her to get out socializing.what do I do?😭

I wouldn't do it, because I'm a deep thinking sensitive fool.

At the same time, as the MIL, I wouldn't have knocked on the door when I could see she was entertaining.

Only in the parallel universe that is mumsnet could so many people think SIL "did nothing wrong" or that she is - in the words of one keyboard psycho-babbler of the kind usually found on these threads - "enforcing healthy boundaries" - what a load of absolute rubbish!! The only possible explanation that might in some way make this seem even vaguely ok is that her family are complete scumbags and she's mortified by them. Otherwise SIL is a rude, callous, socially inept weirdo.
I would suggest to your mum that she now "enforce some healthy boundaries" of her own and drop the visits to SIL until she gets an explanation and an apology.

I agree, I don't believe it was a snub and she was rather put on the spot when your mum knocked. Maybe afterwards she wished she had asked her in but you don't always think at the time.

It sounds as though their relationship is sound so try not to think about it any more, encourage your mum to do the same.

Er yes. I don’t welcome visitors if it doesn’t fit in with my plans. This is quite normal?

MiL is in the wrong.
She was told DiL was unavailable, but decided to try and invite herself in anyway.
She has absolutely no knowledge of what was being discussed, maybe someone could be upset or down.
Maybe the son doesn't want MiL meeting her family? Could be any reason.
MiL is being rude.

I'm amazed that in four years the two sets of parents haven't met - I think that's very bizarre. I can completely understand why your mum rang the bell when she arrived and could tell there were people in the house; she presumably thought that the sister in law was going to be out when she said she wasn't available. Under the circumstances, I think your sister in law was extremely rude and it wouldn't have hurt to have invited her in for a while.