Do you just accept or decline invitations from family or do you ask if you can bring someone or have a conversation about occasion?

Agree with your SIL. If you are doing the hosting @demesneDame you can have all the open doors for anyone to come without an invite, when someone else is inviting you don't add to their guest list for them. Decline if you don't want to go without the newish partner and their kid.

Our family has an open door for some things such as some snacks are provided at a party everyone brings their own drinks or strict invite only for others such as lunch or dinner.

So if they invite you say “sorry we have granny over that day” or “sdc is here is that a problem to bring home” ringing and asking if you can bring anything is totally normal

Im pretty laid back but it sounds like its not a one off situation that you're bringing extra randoms with you

On Sil's side here.

It's one thing a couple of weeks before saying, as someone did upthread, "bil is staying that week, do you mind if they come too, otherwise dp will stay at home with them" compared to phoning on the morning and asking to bring more people.
I suspect they're doing written invites to try and hint that it should be just the people invited.

Your dh fetching his child and then asking for a sandwich is definitely cheeky unless you'd okayed that before. They've done lunch for you all, relaxing then there you are asking to make food for someone who wasn't meant to be there?
You should have either all popped off at 4 with a cheerful "must go now - we have to pick up dc" or let them know beforehand "we've got to pick dc up at 4, but dh will do that and then be back". And if they needed food you should have picked up a meal deal on the way.

It sounds like that sort of thing happens regularly so not just a one off where you might roll your eyes and assume arrangements got mixed up.

Me and my dc just ring up and arrange to meet. I haven’t had a formal invite ever. We would confirm if it’s an adults meet up or family. If my sister was visiting (dc aunt) I’d bring her but they would know she was here and expect her to come.

Now you updated I think most people would think your family are the unreasonable ones, sorry.

If you come from a family where you enjoy the chaotic “the more the merrier, everyone welcome” kind of environment, I can understand why you won’t understand your ILS point of view.

However, many people don’t live like that or enjoy it, and need things to be super orderly. It’s a difference, doesn’t mean anyone is wrong.

But surely SIL knows that your weekends for e.g. can't always be child free, and you may have to play mum/dad's taxi for some of it?
So you get an invitation and say "I'll see you at 1 but just so you know I must go and pick up DS from his walk at 4 and I'll need to take him home or if you prefer I can bring him back but he'll need feeding so let me know if that's too much of a pain, it's no problem if I just take him home".

I don't have adult DC so they wouldn't be hosting me! But my sibling will say if a suggestion doesn't work, like my example of the accommodation above.

If my family are inviting me to something I automatically assume DH is invited - as does DH when his family invite us to things. My family basically have an open house policy though - the more the merrier

SIL is right. You either accept or decline and if you accept you don’t change the goalposts. Your son gets his own sandwich. As on one off it’s probably not an issue but as a regular thing ringing and adding people on expecting catering for extra people is not on.

And I’m sociable person who likes company but it would piss me off if done repeatedly.

Given the context I think it sounds like your family’s been a pain in the arse.

When my sibling invites us over it’s clear that the invite is for me, DH & DC, and I confirm in advance exactly who’s coming (very often just me, as DH & DC stay home with the dog).

If I have someone staying with me I decline the invite rather than asking for my houseguest to be included. I certainly don’t spring any unexpected guests on anyone at the last minute or ask anyone to make a sandwich for someone I’d originally said wasn’t coming!

This!
I only have one sibling and they would make it plain if they could put us up ("it's really tight as both the kids are home from uni") but would expect any of my family to come for a meal ("I hear you're in town next weekend, please come and have lunch, is it all of you or just you and your eldest?").
This would all be via WhatsApp or if part of a conversation with my elderly dad who doesn't do WhatsApp, would be via email (e.g. first set of arrangements fell through but email chain continued or sibling wasn't clear if dad was staying with us and travelling on with us or not).

I can see why some people might not like to be asked about other people coming if the wording of the invite was very specific, although I also think that's being kind of weird as a host and you ought to be open to partners or kids for example, if not necessarily grannies or others.

It sounds like you're maybe imposing a bit but they are also being on the inflexible side, so everyone is being a bit unreasonable.

It's different family dynamics. I assume this seems normal to you and your family but like SIL I would find it annoying. It's a shame however that your mum overheard and got upset.

I definitely wouldn’t assume I could bring anyone else without asking.

I'm organising a family party for ds's 21st.

So far my brother has said that his wife's brother is visiting from overseas and that's the weekend they've planned to spend with him so can they bring him? They said up front they would pay for his meal. If not bro will come to the party and his wife will stay with her brother. I said yes they could bring an extra.

My cousin has asked if her 14 yr old can bring her boyfriend, again offered to pay for him. I said yes to that but with less enthusiasm. But at the end of the day she's the sort of kid who will be awkward by herself so she'll have a better day with him. I imagine they'll eat then go out for a walk or something.

Another cousin has asked if he can bring a newish partner and her 2 kids under 4. No-one has met them yet, the youngest kid invited will be 13. I've said no it's not a huge space, there are no other young kids and there's nowhere for them to run around. It's a sit down meal in a fancy place ds wants to go to. Then drinks at our house but our house is already quite small so 2 little kids running around doesn't seem fun.

Still waiting for other requests. I'm sure they'll come.

Do any of those scenarios fit yours?

I guess the issue is. Who has been omitted from the written invitation? Is it someone living with the others who are invited?

I don’t know anyone without access to a word processor. MS Word or Google Docs are the ones most frequently used by people I know.

I’m impressed they still have a word processor! My parents had one in the 90s, didn’t think they were still going. Is the menu retro too?

I'd just accept or decline.

Are you able to give your actual scenario rather than some cryptic question?

Who is sending word processed invites for a casual get together?

I suppose a ‘normal’ response would be along the lines of:

‘Hi Joanne, thanks so much for your invitation. I’ve already promised to spend the day with DP and his daughter/granny, so I’ll need to regretfully decline - unless you’d like us all to come? Let me know what works for you, I won’t be offended at all if you can’t manage three of us! Happy to bring contributions, and speak/see you soon my darling either way xxx’

What's the occasion? If it's someone's birthday then I would ask if invitation extends to other people in my family (which, honestly, I won't need to because the invitation would be for the whole family anyway). If it's "hey, do you want to pop round for a coffee on Tuesday" then it would be "I am meeting with Granny on Tuesday, we can come to you or join us in town for lunch".. Not really something I ever though about. Nobody is sending me RSVP that I can accept or decline.