If your parents and the guests who arrived after you hadn't seen each other for some time, perhaps they were just engrossed in talking to each other. Also, if any of them were the type to launch into long monologues, where everyone else just has to sit and listen, ( I know people like this!) rather than engaging in to-and-fro chat , it may have been difficult for your mother to intervene with a suggestion that everyone moved through to the sitting room.
I agree, though, that after you went through to remind them of your, and your daughter's presence, one of your parents should have ushered everyone through to the sitting room where everybody could have a seat and enjoy each other's company. After all, the whole point of the occasion was to socialise, and you and your daughter weren't being given the chance to join in. I don't blame you for feeling a bit hurt and left-out.
If, as it sounds, you were left waiting for 20 -30 minutes on your own, I'm not surprised that you began to feel that it was a bit pointless to stay - after all, no-one was showing any desire for your company, and it was a humiliating position to be be put in. You decided to vote with your feet and I don't think you can be criticised for that.
I'm sure there was no deliberate intention to hurt you or leave you out of it, but no-one (brother and sister-in-law. and guests included) showed consideration for you and your daughter so, equally, nobody could blame you for leaving.