Yikes that's a tough one. I see you've been on Bluelight for a few years so you might feel comfortable here, but there must be some great Alzheimer/dementia specific communities out there with a lot of people who have gone/are going through this exact thing which might be more helpful.
Obviously I'm not privy to your family's inner dynamics, but if it were me I'd mention it to my mom right off the bat (with all the "probably nothing" joking and typical "because I love you" jargon) incase things progress then a base has been established. With something like dementia (even disregarding early intervention as more successful in treatment) I could imagine it being a lot harder for the patient themselves to deal with a diagnosis further along if they have deteriorated. As for the sisters, again it depends on familial relations, but I definitely understand the concept of protecting the younger sister and not worrying them unnecessarily. Conversely I also can't imagine having to go through something like this without presenting a united front with support from my sister if there were a serious issue.
But enough doom and gloom and let me get back to your question.
Should I be worried about this?
I think you clearly already are worried about it and it all comes down to how you deal with that worry. Whatever you decide you are obviously struggling to make the best decision for everyone involved and nobody will be able to fault you for that. Frankly even a trained professional probably wouldn't be able to rule anything out or make a diagnosis at this point. However a trained professional - perhaps just your mom's GP who would be aware of the family history - will know the proper protocol to follow and how to check through the criteria that may give some indication.
It very easily could be absolutely nothing, it very well could be something, and it could even be that the events you noted aren't relevant but with her age and family history it is still prudent to begin some sort of testing/monitoring. Anxiety and worry stems from indecision and lack of direction, while peace of mind comes from acting on any plan whether or not it is the best one. Posting here was a good first step, and talking to someone with experience (as in an Alzheimer's specific site as I mentioned before) is a good second one. Whatever you decide to do after that though make sure that the course of action is well defined and you stick to it as opposed to letting uncertainty linger on. Choosing to let it be is still a choice and better is 'let it be until I note 2 more instances' or something like that. Again there is established diagnostic criteria about how many instances is normal that you could find out from a professional.
My apologies if I said absolutely nothing helpful with respect to if you should ignore it or not, but that was by design. Nobody here is in the position to diagnosis your Mom, and I hope nothing I said influenced the eventual outcome of your predicament too much one way or the other as in the end it will be on you.
I felt though that you just wanted to lay it all out and hash through it which I hope I've helped do.
Best of luck,
kill your tv!