Worried about SIL

My SIL and I have sons only a couple of weeks apart. During our pregnancies, we (and the rest of the family) would joke about how different SIL and I were when it came to our outlooks on pregnancy. I stayed away from the no-no foods, exercised as best as I could, read all the books. "K" constantly obsessed about cleanliness (wouldn't pick paper off the floor of her cubicle if she just washed her hands. If she couldn't get a co-worker to pick it up for her, she'd grab it, then wash her hands again.) K would tell waiters at restaurants to make sure her meats were well-done, a little black on the outside was fine, but then worry that the waiters/cooks would do something to her food because she gave them a complicated order. We'd laugh it off, but since it was our first babies, didn't think too much of it.

Our DSs are now 5 months old. Ever since K returned to work, her attitude and demeanor have completely changed. Our dad watches the boys during the week while we work, and she has irrational fears that a) dad is not going to feed DS, b) dad is not going to clean up after the boys so they will swap germs, and c) my older brother (whom she has always hated for no particular reason) is going to molest her DS and my dad is going to let them do it. My dad would NEVER allow anyone to harm our sons. My brother would NEVER harm any child, especially his nephews. She doesn't want my bro to touch/speak to/be in the same room as her DS, but he helps my dad with MY DS out of respect for her feelings. Dad once asked her what K wanted him to do if Dad was holding my DS and he started crying, and K said to give him to my bro and Dad would pick up her DS. Dad said, "you think D is a perv, but you would be OK with him touching G?" Her response was, "G isn't my son." Like I said, my dad and bro are SO FAR AWAY from perverts this is totally irrational. She has admitted it is irrational, but she won't speak to her doctor about it. Not only does she have delusional fears about my older brother, but she won't let my younger brother (her DH) hold her DS, feed her DS, change her DS, or take him anywhere unless she isn't there. DH has to keep track of all his feedings, poops, etc and let her check the notebook when she gets home.

Am I wrong to suspect postpartum anxiety disorder, postpartum obsessive compulsive disorder, or even postpartum psychosis? We've pleaded with her to speak to a counselor, but she won't. My bro is doing everything he can, and the rest of the family is doing their best to a) follow her wishes that aren't entirely irrational, b) prove to her that her irrational fears are not going on and she CAN trust us, and c) give my bro and her DS all the emotional help we can. We absolutely LOVE SIL (in our family there are no in-laws. If you marry in, you're one of us!) and we don't want her to harm herself, the baby, or runaway/leave. What should we do???