My father-in-law's strange behavior is making me feel trapped and I am afraid of what will happen if I tell my husband.

I'm glad you came here to vent. This is a safe place for you to come. I'm sorry your life is largely lived within this hostile environment. I suppose your husband will be upset when he hears the tape for multiple reasons. As you say, he already knows there is a problem and it might force him into the hard decision that he has to do something with his dad besides leave him home all day with you. Is his ex-wife your husband's mother? How long were they married? Would you be happier if you worked outside the home? What do you want your husband to do about his father? Do you have friends or family in to visit? Can you expand your comfort and peace of mind by enlarging your life outside the home? Are you expected to care for your FIL? Do his laundry, make his bed, give him meds, make his meals? Are you able to get out and walk each day? This can help shake it off.  Hate is such a strong word and emotion. Be careful and take action like coming here to diffuse this awful emotion within your spirit. It is harmful to your health. Coupled with the feeling that your husband would defer to your FIL instead of you, makes your hard feelings especially toxic. An intensely painful way to live. As you increase your confidence and self worth, you'll feel better and stronger for when you have the conversation with your husband. 

Yes, tell your husband. Don't bottle it up, because your resentment will only come out later in some form, and then he will wonder why you never told him you what you are feeling and going through.

My FIL has a different problem than yours -- he's sexually inappropriate to me -- but he also hides what he does/says when other people are around. At first I didn't say anything because I feared no one would believe me! I did tell my husband after a few weeks and he did believe me, though I don't think he or his sister fully comprehended just what I was talking about. But they knew I wouldn't just make up this stuff. Finally I decided to start taking video with my cell phone. FIL has dementia and doesn't know he can be taped. Anyway, a couple of videos later, my husband knows exactly what I'm going through. We devised strategies to deal with it and I know if it ever reaches the point I cannot abide taking care of the old man, my husband will have my back. You may be able to devise new strategies for your situation.

The important thing is your marriage and your trust. It's best to bring your husband into this discussion and find a way to deal with FIL as a team. Your husband is counting on you, also, to help care for FIL, so don't feel you have no agency here. What you think and feel is important. This situation cannot work without you as part of the caregiving.

I agree with CM. You're being responsible. I just wonder if you understood a bit more about his illness, it wouldn't feel so dangerous to you.

This is a lifelong pattern of behavior. He MAY need a higher level of care and/or meds.

But yes, be honest with your husband about what is occurring and how it is affecting you.

How do you think your husband would feel if he later found out about this and you hadn't ever told him about it?

I realise it's difficult. See if you can imagine doing this: say to him, I need you to listen to something and just tell me what you make of it, please. Give him the audio to listen to. Give him time to think about it. See what he says.

But meanwhile, and in any case, you and your husband and your SIL need to think ahead and (in their case, less so yours because he's not your Dad) perhaps think a bit more deeply about what kind of care and support your FIL needs.

When a person is clearly... not well, it stops being a question of hate or dislike or 'rubbing each other up the wrong way'. He needs skilled support from people who know what they're doing. And for your husband, loving his father doesn't have to mean having him in the house and making his wife's life a misery half the time.

There's nothing *wrong* with telling the truth about this, is what I'm saying. There's a lot wrong with trying to pretend it isn't happening - that's not going to help anyone in the long run.

So don't think you're being selfish or hateful. The opposite. You're being responsible.

Are you saying FIL has a diagnosis of Schizoid Personality Disorder? You might want to read up on that, perhaps on the Mayo Clinic site.

Is FIL receiving any help for this illness? Is he followed by a geriatric Psychiatrist? Is he taking any meds, such as antidepressants?