i think it’s a little rude of him to not ask you ahead of time so you’re on the same page. it would’ve been better so that you can let your family know you’re bringing someone! especially telling her to just stay over without checking in with you first. if it were me, i wouldn’t make a big deal of it this time, because it’s a family holiday it should be fun! but i would let him know that in the future you’d like to be on the same page, and talk to each other first before inviting his sisters or anyone else along to your plans
Just let him know that you will ask your parents if it’s okay so they can make room for one more child for the egg hunt.