Have they booked tickets already? If not, can you call sis and ask her to split the trip and come later with her family? You can tell her that the amazing stuff you want to do with her won’t be possible with mom around. Mom’s trip is probably to stroll buckinham, go to the national gallery etc which her baby won’t let her enjoy anyway and she might have to stay home with him. It’s best she comes later and you guys can go together to a ton of places with a lot of walking around and seeing things that mom won’t be interested in. Come up with a list of things mom won’t want to do that you can do with her and won’t be able to if she comes along with mom.