What a bummer Angeline.
I had the same problems with my grandsons. I could only handle picking them up till they got to be about 15 lbs... so needless to say, I didn't have much "bonding time" with them. Fortunately my husband was very understanding (and hates it when I hurt my back or go into a flare) so he would do most of the lifting and then put them on my lap.
I'm extra sorry that your bro. doesn't get it and that you feel that you have to tell him to back off. When it comes to people who don't believe in our pain I like the quote that was floating around yesterday. "My pain is more real than your medical expertise" or something like that. (I thought I had saved it but alas I spaced out). I also remind them that fibro feels a lot like the worst flu they ever had, when it comes to achiness, pain and spaciness. If they don't get it then I like to treat them, when they have the flu, the way they treated me on any given day of fibro. Is that mean?? yes, but as much as I hate to say it.. they deserve it.
I'm glad you love your neice so much and hope you continue to be a big part of her life.
Take care,