Shall I leave my boring husband to be a single mum?

I like how you say you're not "allowed" to go out with your sister. There shouldn't be conditions like that in a marriage, you shouldn't ask for your husband's permission to be with your own family. He clearly has some issues, the site he logged on as single is reason enough to at least talk about things with him seriously. I personally think these situations and behaviour need honesty and saying things openly and straightforwardly, without "pampering" the person. Tell him that you've had enough of this and he may describe your sister as he pleases and not like her, but you don't have to stop seeing her because of what HE thinks.