I'm married but I think her sister wants me and I don't know how or if I should tell my wife
This is unusual. Why do you think her sister is into you?
I'm not sure, she lives with us and I saw her checking me out while she was doing her chores.
I can see your concern, but it is wrong to tell people about something that will hurt them. You should first check with you wife's sister to see if she's really into you and figure it out. Otherwise you might end up ruining a relationship between them.
So I should lie to my wife and not tell her.
Not telling her won't be a lie if it's not true. If you verify that the attraction is real, then you can consider telling your wife. Until then you should keep the thought to yourself, so you don't cause problems for nothing.