Is it normal to want to get it on with my wife's best friend

It's normal for a guy to want to have sex with an attractive woman. Only a guy who thinks with his dick will act on such impulses without regard for how that will affect others.

It's very likely that in the friend's mind, you're completely off-limits due to your wife being her best friend, so there's no danger in her flirting with you.

Also, you say she "kind of flirts" with you. Are you sure you're not seeing flirting where there's nothing at all really going on?

When I injured by back on the job I asked my younger sister to take my place as a wife and she gladly agreed. Since the two of them had always got along it was not a problem for either. All worked out fine and she moved in.

Hot.

I always try to fuck my wife's sister but only way is to let her and sister strap on fuck me and be a sissy for them

No.

It's cheating.

Stop.

If she touches you and flirts she must really want you

Flirting is just harmless fun with friends you feel comfortable with and not to be taken literally. I know a few people who have read too much into direct and even outrageous flirting and ruined their lives. As already been pointed out, she only, "kind of flirts with me sometimes".
Hardly the grounds to cheat on.

Flirting is there to turn people on. But don't act on it and run two families at once. Especially if one of them have children

Don't cheat on your wife with the worst possible person you could choose, don't cheat on your wife with anyone. I think it's best if you don't flirt back with this woman ever. When she flirts just give her a blank stare.

Flirting can be just harmless fun in her mind.

Have a threesome.

Some times she'll ask us to come over more often then she'll look at me and bite her bottom lip