Theresa is NOT a wimp, she is just smart enough to realize that Ethan NEVER believes her when she tells him the truth. Plus Ethan and Gwen took Jane from her and wont give her back, so WHY would she tell him bout lil Ethan cause he will try to take him away from her too.
Luis, he loves Sheridan but she is married and keeps telling him to move on. So he is, with Fancy and it is about darn time. With Sheridan he might as well tell the wall about Chris cause it would believe him more than she does(and it listens better). Luis has his gut instincts and he follows them, he saved Fancy and Sheridan(on numerous occassions) so he is The shi_t!