Oh my gosh, your post made me lol. Not in a funny way, in a head shaking way. Just tell her that you guys need your private time. She needs to respect your house and your wishes. If she can't she will have to go.

It sounds like her behaviour may be related to her mental breakdown/mental illness. I would suggest you and your bf/husband discuss ways to talk to her and establish boundaries with her. I think that he needs to take control of the situation and initiate the talk, although obviously you need to be a part of it. I agree with previous poster, if she cannot respect your rules it is time to find her somewhere else to live. As for the lock on the door, tell her that you enjoy making love with her son and do not enjoy the idea of her walking in on it! LOL I truly hope that this is a temporary arrangement, there must be somewhere else she can stay. Also, I really hope that she is seeing a counsellor or psychologist. Good luck and remember to be firm when discussing boundaries.

Janine makes a really important point - your MIL's behavior could be very much related to her illness. I'd encourage you to make an appointment with your MIL and her doctors so that you can fully understand her condition and the symptoms of that condition (of course, you'll have to involve your MIL, or get her to authorize her doctors to speak to you in writing in order to get her docs to speak with you).

There's something you don't mention, but based on your MIL's truly boundary-less behavior....did she behave that way when she was raising your fiance? Or was your fiance raised by someone else? Or is this bizarre behavior a recent occurrence with her? (You don't have to answer - I'm aking in order to highlight a potential consequence for you and your marriage.)

The reason I'm asking is that it's hard to imagine someone as dysfunctional (as your MIL sounds) raising a child who has healthy, functional boundaries. I guess my concern is that (before you marry), you and he really need to sit down and discuss how you'll handle any future crises with his mom....you need to be very clear that when you marry, his primary commitment is to you - not his mother. That may sound harsh, but it's the truth. I'm certainly not suggesting he abandon her, but there are all kinds of ways to help that don't involve her living in your home on any long-term basis. Trust me - you don't want to have a third person in your marriage (and I mean marriage, not home). Please think about working this issue out with your fiance now so you don't take up permanent residence in the middle - between him and his mother. Good luck.

ITA with Janine. Your future m-i-l is acting exactly like my 4-yrs-old son. And yes, we still LOCK the door despite his protestation (he just LOVES to barge in in the middle of the nite, gods know if we're doing adult stuffs in there).
Your husband-to-be needs to be the one setting the boundaries (at least talk to her, you can't be the one talking to her yet). He needs to also see if she should be talking to a mental health professional too or her doctor because of her behaviors, so find out too if she's been like this all this time or is this a new behavior she's suddenly introducing?

Good luck and remember to be kind but firm too (remind your husband) when discussing to her about boundaries.

Nevaeh got it... OP, you shouldn't be doing any of the talking... your hubby has to tell his mom how it is. If she refuses to listen, I'd suggest implementing a "no clothes" rule for your bedroom time with the hubby. LOL! That should teach her to wander into places she knows she shouldn't be! Good luck, and remember, annoying as she may be, she's still family!