"AIBU To ask my mums new gf not to buy things for my baby??"
I would not speak to the gf about it, particularly as you have never met her, but I would ask your mum to tell her not to buy presents.
It is totally weird for your mum's new gf, who you haven't met, yet alone know, to buy things for your baby.
It would be totally weird for your dad's new gf, who you hadn't met, yet alone know, to buy things for your baby.
It would be totally weird if your mum had a new bf, you hadn't even met, yet alone know, who bought things for your baby.
It would be totally weird if your dad recently came out as gay and his new bf, who you hadn't even met, yet alone knew, bought things for your baby.
The relationship is very new and it does sound as if it is being foisted on your siblings. MNers wouldn't normally recommend new partners being introduced to their children so early on. I am not surprised you are worried about your youngest brother. It must be unsettling for him to see his DM with someone new who isn't his dad. Could you have him round to have a chat (without leading it) about how things are?
Whoever your DM introduced to the household at this early stage is bound to affect young children and change their world.
I think you could meet your DM's gf, if you wish, and start to get to know her but I would probably only do so a few more months down the line if they think the relationship is becoming serious. (Same as if your DM had been straight and met another man).
I definitely would be put off by someone trying to ingratiate themselves with me by buying stuff for my baby.
I wouldn't worry about the photos thing at this stage at all. Don't anticipate problems before any are here.