My dad died 2 months ago. My mom is telling bizarre stories that never occurred as if they’re true. Is this a sign of dementia?

Snoopycharlie, my heartfelt sympathy to you and your family on the passing of your Dad.

First I would check with Mom's doctors to see if any meds she is taking for her pain, anxiety/depression could be the root cause. With grieving one can go through all types of conditions, and with your Mom losing the love of her life, this has thrown her into a tail spin.

You may want to have Mom tested for an Urinary Tract Infection as that could cause what you are seeing today with Mom.

Time for a family meeting, without Mom, to see what would be best for her. Staying alone at home wouldn't work, as fear comes into play depending on her dementia. Today, there are so many options.

I noticed on your profile that your siblings and you are willing to take Mom into your home. I highly suggest you read up on Dementia and the different stages to see if you or your siblings would be able to handle this. https://www.agingcare.com/topics/8/dementia-behaviors

It could be confabulations, but I agree with freqflyer it's time to get her to a doctor and possibly a neurologist.

Good advice here. Don't rush in and move her to live with you or a sister, and don't go to live with her either, until you, your sisters and your mother have had more time and more medical input to work out what is happening. Another thing that might be possible (it happened in my daughter's inlaws) is that your father was covering for your mother, and that is why your mother's 'deterioration' all seems so sudden. You need to know more before you can work out what is the best option for all of you. What a difficult situation, on top of the grief you must all feel for your father's death. Best wishes to you all.