Do you really feel bad though? It doesn’t sound like it. I feel sorry for him too. And them. Massively.
Your sister in law was stupid, thoughtless and possibly even calculating in telling you your daughter could come - but did she say to the wedding? If she did, she has shifted the goalposts, and you have every right to be pissed off. If she said she can come with you, as in over there while you attend the wedding, because she has arranged childcare, you should have probably clarified that she would be allowed to attend the wedding also and that family would look after her while you were involved with the ceremony etc. I’m surprised you didn’t check this with your husband’s parents before you left. What’s more surprising is you telling her to actually fuck off right before her wedding day!!!
Even if she’d got you there under false pretences you obviously don’t give a shit about your relationship with her, or your PIL. There were other ways you could have shown her suggestion was inappropriate/ she was out of order without being openly abusive and aggressive.
If she doesn’t have kids and is pre-occupied with her wedding (as most brides are) she probably didn’t give it a load of thought. Surprisingly, she might have had a litany of other logistics to focus on besides you. She probably thought she’d come up with a solution which allowed you all to go etc. so she could have her brother share her big day, you’d be involved and she’d get to spend time with her niece before and after and that you’d actually be grateful. Obviously it was the wrong thing to do but you clearly resent her and her parents anyway so you were bound to react even more negatively to what was a silly, impractical suggestion.
Your husband obviously is well aware of your deeply negative feelings towards his family and is making allowances to accommodate you. Just what have they done to make you hate them so much? Is it mutual? Clearly not as his sister asked you to be her bridesmaid FGS - bet she regrets that! Does he get on with your family? How often are you allowed to see them? A considerable amount more I’m guessing, presuming they live in the UK.
What if, when your daughter is older and meets someone - perhaps travelling, like you did, or not, and they happen to take a profound disliking to you, would you be happy at the minimal amount of contact she (and her baby) would have with you because of this?
I have no idea how your ‘outlaws’ have offended you other than you feel they resent you for taking their boy away* *and I don’t know how they express this to you, but given that you’ve said he loves them and he still has a good relationship with them but has only seen them - once was it? - since your wedding, and you now have a child together - wtaf?! Presumably he would see them a lot more often given the opportunity but he hasn’t. Is this due to finances or bc he knows you detest them and would make it unbearable for him?
You’ve now told him to pass on (RIGHT BEFORE HIS SISTER’S WEDDING) that they are all blocked and you and their granddaughter/ niece won’t be having anything to do with them again until she is older. Do you normally overreact this way? Why have you decided to punish the parents as well as the sister? And what gives you the right to decide this when your husband is equally her parent? Presumably when she’s older that won’t happen anyway as you’ve successfully alienated her father from them and with NC she’s only going to have your side of the story as to why her dads family are a load of despicable cunts and she’s better off without them. Seriously??
Have a word with yourself, OP, and GROW UP.
Believe it or not, this isn’t all about you. If you really love your husband, suck it up and try to make this trip as easy and as pleasant for him as he no doubt would for you seeing your family, that is if you’re still on speaking terms with them. You sound alarmingly selfish and high maintenance so I wouldn’t be surprised if they’re also banned or walking on eggshells as well.
Who knows when he will see them all again - if one of them suddenly dropped down dead before he does how do you think that would make him feel towards you, knowing you have put a stop to him (and his daughter!) seeing them through your petty, entitled, selfish attitude? It’s not like you have to see them every week, month or even year!
You’ve spoilt this trip and ruined what should have been a wonderful family celebration bc you didn’t clarify the situation and you’ve overreacted to a thoughtless suggestion. Nobody has died. Anyone would think they have purposefully ruined YOUR wedding day with the way you’re carrying on.
Get over it and be kinder to your husband and daughter - they’re her family too.