I agree with Liz. Which country are we talking about?
When my mother died she gave power of attorney to a sibling, so I presume all photos etc my mother had would have become the sibling's property.
Not necessarily. In the UK power of attorney is given to someone usually long before a person dies, so that they can manage affairs and money if the person is unable to for any reason. It doesn't mean that they automatically inherit from that person (POA can be given to a solicitor and obviously they wouldn't inherit.)
You need to find out what your mother said in her will - if she left everything, including her personal belongings to your sibling, then as Liz says the best thing is to write to them and ask. I don't think there would be any point involving the law, as I can't see there would be anything they could do. However, if her will says it had to be shared and it hasn't then you need to contact a solicitor for advice.
If you are in the UK you can check here to see if she left a will
https://www.gov.uk/search-will-probate If she died intestate you also need to seek advice as you would be entitled to an equal share.
Be prepared for disappointment though, as if your sibling isn't interested in Family History, they may well have destroyed them when they cleared the house.