My question is related to family.
My brother in law asked my husband about the 2 of us coming to sydney to surprise their mum for her birthday.
As their mum and dad would be coming from south australia to visit.
I was raised that if you ask somebody to come from another city or state that you in fact are
Asking them to your home.and they would be welcomed to stay with me .
I asked my husband several times what the arrangements were on where we would be staying .
He emailed his brother the day before driving to sydney and asked if we could/would be staying at their house.
The brother emailed back and said they would be sleeping on the floor and there was no room for us.
The brother also said he had picked a nice place to take the mother and father out to eat.
The brother said he would be paying for his meal and that of the mother and father (for her birthday) and we would only have to pay for our meal.
I may be wrong but this doesn't sound right.
When I ask somebody to visit I put them up in my home and treat them as a guest in my home
Would somebody tell me if I am wrong or over reacting in this event.

Usually I'd agree with you but if there is no space there is no space and there is not much that can be done to alter that.

Not having the room to accommodate all that your brother-in-law wants to have visit isn't such a big deal.
The mother's travel and birthday are the key topics.
The invitation should have been made far enough in advance that arrangements for suitable abode could easily been made early.
They want you to be there to particpate. Just go and enjoy the celebration.

I don't think it makes sense to presume that letting siblings know that they are planning something for their mutual parents, and would like everyone to be there for the benefit of those parents, is really the kind of invitation that requires the planners to put everyone up or feed them. If this were the case, a lot of families couldn't get together because the expectations would be hard for many people to meet.

I lived 400 miles from my family for many years. I knew I could always stay with my parents, as they had extended a standing invitation to me and my small family and had room for us. However, other's also invited us to things in our home state frequently such as get togethers at restaurants, christenings for their children, weddings, family reunions and more. These invitations normally required that we pay our own way and arrange our own accommodations.

Unless the invitation is, "we'd love to have you come and visit us, we have plenty of room!", or "we'd like you to join us for a dinner honoring Mom and Dad, our treat", you should assume that while your relatives want to include you, the expectation will be that all but those being honored will be on their own financially and in terms of accommodations.

In general, I find that families are so different - in one family a lot is implied that is not at all understood or traditional in another. If it's not your own family of origin, I'd suggest that if anything is in question, clarify it. For example, in this case, you might ask, "do you know if anyone might have room for us, or would we need to find a hotel?", or, "what do you anticipate the cost will be for the restaurant?" Asking in this way, you are not presuming any accommodations but rather information.

Hope that helps!