I recently scanned 11,300 family photographs, including albums which dated to the 1910s. I may be able to help you with dating the album and uncovering more inscriptions.
Smell the album. If you can, remove a photo from the album and smell it directly.
It sounds weird, but it will tell you if the pictures are nitrate or acetate. Acetate is slightly newer (1950s and newer) and it has a very distinct vinegar smell when it breaks down. Acetate photographs get a faded appearance when this happens. If the album or pictures smell like chemicals which you cannot readily identify, they are probably nitrate. Nitrate photos will "smear" with age, and develop brown marks that look like cigarette burns.
If the pictures are glued into the pages, you can try to remove them, but the method I recommend will destroy the album. So before you try it, take high-resolution scans of each page. Digital pictures do not have sharp enough resolution.
Use an xacto knife to remove each page from the album. Then use it to cut each page into halves, so you have one piece with two photos, one on each side. Get a tray big enough to hold the photos and fill it with ice water. Place each piece of doctored paper into the tray one at a time. The majority of glues used in old albums are water-soluble. Most photos from older eras were written on in pencil or in water-proof ink. (Was the photo you found of you and your siblings inscribed in pencil, or in ink? If it's ink, test the water solvency of the ink by dabbing some water on the inscription with a Qtip.)
After five or ten minutes, the piece will sink to the bottom of the tray. Gently remove it from the tray, lay it flat on a soft towel, and press it dry with another towel. Then attempt to GENTLY peel the photo from the paper. It may need a couple of soaks before you can remove it successfully. Keep in mind that the water will have to be changed frequently, and that the photos will be covered with glue-water--this is why you should wipe them dry, to help remove glue-water from their surface before it solidifies.
If you are afraid of blurring any writing on the back of the pictures, you and your mother can try dabbing water on the corners of the photographs, drying them with a towel, then working them free very slowly. This is more precise than soaking them, but also more time consuming.
Wear white cotton gloves when you handle the album. This will keep your hands' oils from transferring to the paper or photographs. When you aren't investigating, put the album in a freezer bag and store it in the freezer. This protects the photos from further chemical breakdown (very important if you determine them to be nitrates). Finally, if you are concerned about destroying the album or DIYing the project, I would suggest contacting the nearest well-funded museum or an online archiving service. You can, for a fee, get someone else to date the photographs and possibly get any writing on their backs recorded for you.
Frankly, it's spooky that the album is in such great shape. Unless the old lady was storing the album in an archival box she didn't share with you, it ought to be the worse for wear. Seeing as you found a photograph of you and your siblings hidden in there, it's even more important you get every scrap of information you can about this album.
I hope this at least gives you ideas on how you can investigate further. Stay safe, OP.