Sharing too much information...

Oh, dear. If mom has dementia, you must understand that she is in her own world. When my mom had dementia, in her mind she had been a famous stage actress in New York. She would tell me with the most sincere voice that she remembered me because I had been on stage with her and in some commercials. She also repeatedly told me that absolutely everyone in the facility was having sex with everyone else. Men were stalking her and hiding in her closet.

Educate yourself on what happens when someone has dementia. How they act, the things they say, etc. People affected with dementia say outrageous things, and most of it isn’t true. I finally got to the point that I told Mom to be a lady and that ladies don’t say things like that.

I would try to ignore what she says about embarrassing and private matters. Much of what a person with dementia says is not true. They have delusions or remember things incorrectly. I'd try to just let her talk and change the subject. I don't know of any way to prevent it. Eventually, it may pass.

If she has dementia, it is entirely possible that she is having a delusion or experiencing confabulation. It's part of the disease and you should not believe what she is saying. Validate her feelings and redirect: "Mom, you always had an interesting life, remember that time when . . . "

Please share with your brother that these kinds of "memories" are created by dementia, and neither of you should accept them as objective truth.

BigSandwich, excellent answers above. Here is a good article regarding this subject that I found here on Aging Care.

https://www.agingcare.com/articles/playing-along-with-dementia-realities-121365.htm

Two things may be going on...
1. Dementia has a tendency to drop filters that we put up. Someone that has never used a curse word will all of a sudden make a sailor blush. Someone demure and proper may become very sexual. Someone may begin stealing things, shoplifting.

2. Imagination, things may seem real and if it is from a book we have read or a movie we have seen or a dream we had all these can then become "reality". the neighbor that has helped mow the lawn and taken in mail when you were on vacation now becomes a "thief" or a past lover....

It is best to try to deflect the conversation, do NOT argue. You will never "win" an argument with a person with dementia.
If in fact these things are true it is hard to "un ring a bell" so if you can find it in your heart to forgive the past and move forward you will be better for it. If your Father is still alive feel for him that he also carried this burden, if he has passed and did or did not know about this again he must have had a heavy heart but the fact that he kept this to himself he was/is a strong man. If he could forgive you can as well.
Do not let this be all that you remember about your Mom.

as jjariz said - it may not be real. your mother could be relating stories she read - many women read 'spicy' novels and with dementia comes the confusion of what is real and what is imagined. and yes, they even remember things incorrectly sometimes.

Was your mom someone who watched a lot of soaps? or read romance books? - if so she has mixed the women with herself - it is common & a good reason to deflect people with dementia from watching crime shows as they think 'the bad guys are outside right now'

My mom once out of the blue said 'your father is a real bastard, he wouldn't even go to his own brother's funeral' - to which I said 'yes he did & so did you & the reason I know is that I went too - mom you even did a reading'

Try TEEPA'S GEMS videos on line - this with help both you & brother - your knowledge is the best way to cope

Oh my gosh! Do understand that none of this may be true. Under the broad umbrella of mental illness, there are many conditions classified as sexual disorders. They are pure fabrication and hold no basis of truth. When she is telling you these things, do not respond. Don't get wrapped up in her tales, albeit difficult to do. Think of it as the elder mind playing tricks on her. Especially at nightfall, elders' minds wander!

The person with dementia will tell all kinds of stories. When we would go past a motorcycle, my mom would say she had a motorcycle. She would even turn to my step dad and ask where it was, hahaha. I can tell you she never rode a motorcycle. She wouldn't even sit on my dad's when he was a motorcycle police officer. She also would talk about riding horses.... She never did that either, but I had horses and ride as a teen. They just mix realities. Chances are, none of that ever happened. Who know why they say these things.

Maybe it's just they might have wanted to ride a horse or motorcycle or be less lady like but were too timid - now the governor is off the timidness and the mouth goes in gear - a little [sometimes] harmless fantacy -

We all have had the daydream to ski a hill like a pro or sew the perfect dress but didn't have the skills so now our dementia friends are saying they did what they never could but maybe wished they could - it's a kind way of looking at it & if it is not harmful let them be - asking details will only show it is a fantacy

The girl who was a wallflower or quite uptight may have fantazied about being the most popular & substitutes what they think would have happened for the more humdrum that they actually lived - who knows what lurks in the minds of those with dementia