Seriously OP, what the hell do you think somebody means when they say NO ?
"The invitation was sent from girlfriend’s phone and said “Can you come...” I just thought it was a meal in their newish house. I replied that me, partner and his two teenagers could come. The response came from girlfriend “Sorry can’t accommodate Partner and teenagers but look forward to meeting them another time.”
This was a NO you ignored it. That was very rude and you should have listened and complied.
"I was a bit put out and replied that the kids were great, well behaved and would fit in. Again response was sorry they could not be accommodated."
This was the second NO which you ignored. You should have listened and complied the first time.
"I now regret texting my brother he said that they hadn’t entertained before so didn’t really want extras."
This was the third NO
He did hint that maybe my partner could come alone as he and girlfriend hadn’t met his kids. Only because you had battered him into a corner and he was trying desperately not to ruin his big day because three times previously you had been told NO and did not listen
" I was really upset as my entire family were going to be at this meal but I couldn’t take my partner and his lovely girls. So then still interfering, you rang your Mother to pester your brother on your behalf
My mother then spoke to my brother and he rang me up annoyed that I had mentioned it and again stressed that there was no room. "
This was the fourth time you were told NO
You then went and wrecked their big day by barging into their garden with people you know were definitely not invited.
It would have ruined their surprise if they had told you beforehand what was going to happen, all they wanted was their own close family there and that included you.
But you decided that wasn't good enough and you were going to turn up there with your partner and his two girls despite being told NO FOUR TIMES and you completely ruined their day.
You seriously need to learn to listen to what people say to you.
You seriously need to understand that No means No the first time someone says it
You seriously need to apologise to your brother and his girlfriend, your actions were ridiculous from refusing to accept the first No.
Tip for the future, if they get married, read the invitation very, very carefully to see who among you, your partner and his two girls are actually invited and make it perfectly clear only the person or people who are invited will attend. Do not question the invitation, do not try and have your partner and his girls included if they are not named on the invitation.