Steve: Kinda depends on how young the sister is. If she's in her teens, I'd try to join your husband in helping his sister. If she's in her twenties, that becomes creepy. I'd have the three of you hire a psychologist to discuss what the problem is and how to solve it.
Mia: If she's in her twenties, take Baby Sis to the mall for a makeover and help her put a profile on an online dating site. She needs to find herself a boyfriend. If that doesn't work, the next time she tries to sit in your man's lap tell her to move. Do it in a way so that she knows you mean it. If her baby talk continues to get on your nerves, tell her, "Cut the baby talk already! You're 25!" Or however old she is. Stop letting her feel so comfortable in your home. Hopefully, your husband will cosign your efforts and will agree to your limiting her visits to once a week. Baby Sis is not going to like it but that's her problem. Whatever you do, don't let her drive a wedge between you and your husband. If you feel that happening, get a third party like a counselor or a pastor to intervene. Good luck.
Mia: It's hard for people of the opposite sex to really be friends. You can be best buds to a point but chances are, he's secretly attracted to you and just biding his time before he makes a move. Or vice versa.
Steve: "I seen ya I seen ya I seen ya walkin' down in Chinatown / I called ya I called ya I called but you did not look around."