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

Absolutely bang on!

Yes. The SIL is being very accommodating She sets a perfectly reasonable boundary, and now that's an issue.

I'd be much less available.

SIL: I’m busy that day, if you must drop these things off on that day please just leave them on the doorstep as I am unavailable to spend time with you. Thank you so much!
MIL, having peered through the window: knocks repeatedly and consistently until SIL has to stop what she’s doing and attend to the door
SIL, surprised and confused by MIL’s intrusion as she had specifically stated she was not available: Hello?
MIL: Well, I’m here now looks inside and waits
SIL: Sorry but like I say, I am very busy right now, but I can see you another time. Thank you again for the items.
MIL: leaves and cries

It’s easy for OP to say that her mum isn’t intrusive, but at that moment she was and whether she is or not in general isn’t even the point.

Plus, none of us know if SIL’s family would have been comfortable meeting her MIL or her joining them to begin with.

All of this would have been a non-issue if MIL did as she has done before and simply left the items on the porch.

So what was she doing by knocking repeatedly on the door and then getting upset after and ranting to the OP about not being invited in then?
If you weren’t angling for an invite you wouldn’t be upset that someone was busy when they already told you they were busy.

SIL opens door: 'Ah, Sheila! Thanks so much for dropping off these cream cakes like you always have done on the same day so they don't go off - you know how much Dave loves them! Sorry, like I said, I'm a bit busy now with family visiting but thanks again and see you soon for lunch!'
MIL: 'Ah, yes, of course, have a lovely day!' (walks away happy)

Just in case you missed that part..

SiL was with her family and didn’t invite my mum in to meet them. As SiL knows my mum is not intrusive and would not have overstayed her welcome. My mum is so upset but we have stopped her ringing Bro.

If she didn’t angle for an invite why did she repeatedly knock, wait to get asked in, then go home and cry to her daughter and threaten to phone her son complaining because she didn’t get (gasp) an invite?

She KNOCKED repeatedly until the door was opened and then ran home threatening to call her son because she was not invited in....if that is not angling well I don't know what is.

She didn't angle for an invite. That's your projection.

And Sil was still rude. But perhaps you have lower standards.

Exactly.

Exactly.

I'd be a good sum that if the "free gifts" stopped, SIL would only feel relief that she doesn't have to deal with a constant flow of tat any more.

MIL has been dropping things off without acknowledgment before, OP literally stating this in the original post. This is the only time it has been an issue.

Find the “free gift” posters hilarious. MIL is not dropping off gold and frankincense, its bits she gets given free from her work. I doubt the SIL or son cares that much, but allows the MIL this to make her feel helpful and included and it’s a way to stay close.

There is no way these “free from my work” items warrant unboundless gratitude.

You would also think MIL could wait and pop by once a month with them all, but it seems to be such a regular disturbance. So MIL is imposing regularly and SIL is doing everything she can to keep a good relationship there. Even while in her working day will pause to entertain and show face,

MIL is more than dramatic to be crying over this at all.

Free gift - red herring

Sil - allowed to have her own life/boundaries. Sounds like she puts up with a lot probably to keep her dp/in laws happy. Constant visits during working hours etc.

Mil - aged 50 odd! Who sil texted to say she was unavailable. Pushed the boundaries by repeatedly knocking on the door after having a good peep through windows. Then went running upset to her daughter and had to be calmed down from phoning her son.

This behaviour isn't a one of from the mil. I would bet my bottom dollar she's got form for pushing boundaries and everything has to be her way or all hell breaks loose.

Your use of courier is interesting. This isn't stuff the SIL is having delivered to the MIL house or things that she's asked for. Its free tat from work that the MIL is dropping round as pretext for a visit.

Or maybe the son should think his mother himself instead of it falling to his partner now?

I think the SIL sounds like a saint. I find it really frustrating WFH when (often) older people don't get that I am still working and stop by for a chat and a cup of tea. Invariably it takes 30-40 minutes out of my work day. It sounds like the SIL is accommodating this a lot, probably to try and create a good relationship with the MIL. On the one occasion that she has pushed back the MIL has gone running tearfully to another family member and wanted to tell the son?

I would be the son already knows as the DIL has told him. I actually think for the sake of good family relations the OP shouldn't be to sympathetic to her mother here- telling her she's right but to let it go; she should tell her mum she crossed a line and to apologise.

The key thing for me here is MIL knocked on the door a "couple" of times, she was asked to just leave the stuff, she then repeatedly knocked on the door until it was answered. She didn't just think SIL plans had changed so she'd knock to say hi; she knocked, it was ignored and she kept going until she got a reaction.

If I were the SIL this would be a wake-up call that however much effort you put in, and however much you try to accommodate, it isn't going to be appreciated or enough.

I think the MIL should come round and bring whatever these freebies are when her own son is in and stop arranging to come on days that this poor girl is working! She wouldn't be able to do this if the sister in law worked at the office.

What even are the freebies-does sil want them!?

I wonder how MIL would feel if she had nice plans with SIL to do something but SIL brought her nieces and nephews along unannounced. Oh they knocked on as I was leaving

You know, families can funny. I wouldn’t exactly want my default “mum” figure (complicated but irrelevant) and my current MIL in the same room. They are wildly different women, with wildly different values and they would both be judging each other, although both very lovely in their own ways. I would feel very awkward and on edge to have them together for even a few minutes. Don’t even get me started on my strait laced middle class MIL with property in the med sitting down to chat with my motorcycle club running brother. At a busy party or a wedding would be fine, but not sitting in my living room with a cup of tea. I’m not embarrassed or ashamed of either side at all but I also would like to spend time with them each independently for the foreseeable.

If I had outlined I was busy and could not spend time with MIL and and MIL stood at the door expecting an invite I would feel considerably put out. Good on her for holding the previously issued boundary. MIL can be a bit sad about it but that doesn’t mean SIL did anything wrong, she certainly wasn’t rude.

I still would love to know why it's a DIL problem, why is MIL barging on her and not her own son? What is she bringing? Tampax?

Ok, so the MIL drops off stuff for the SIL's 'household' which maybe she doesn't want but her partner does - still pretty rude to expect her to drop and run without acknowledgement. It sounds like it's all a bit of a pain for the SIL so maybe a different arrangement needs to be made.

Yes gatecrash. She was TOLD the night before SIL was not available, she was not told why because it's none of her business. MIL arranged to drop off the "freebies" that she has been doing long before SIL was on the scene so the freebies don't actually concern the SIL in the first place and leave in usual place.

Ohh BUT MIL sees activity. There are PEOPLE there. Instead of leaving the SIL alone to host her own family in peace she knocks repeatedly on the door until SIL is forced to answer to say here is the stuff I said I was going to drop and run because you are busy, expects to be invited in to join SIL family and gets upset when SIL holds her boundaries and does not ask her in, then goes home crying threatening to ring her son and tell! Give me a break.

SIL sees MIL every week, does things with her etc, has been described as a lovely person but now she is the devil incarnate because she was hosting her family and MIL wanted an invite. If I had invited my family to my house to visit and lets face it, nobody knows the dynamics of a family or what's going on and I had already told my mother in law I was not available on that morning and she told me she would drop over the freebies (that precede my time with her son) and leave in usual place and she decided to bang the door down wanting to be invited in she would get short shrift from me too.

It's only settled in your head 😂

I don't think you understand what "being available" actually means

Obviously the SIL had her reasons and she made it clear she wasn’t free to spend time with the MIL.

Honestly it’s absolutely nuts some of you going on about the SILs rudeness but think you can muscle in on someone in their home when they have vocally already made it clear they were busy and just expect them to drop prearrange plans to host you!