That is really young to expect them to sit for anything!
But the earlier you get them used to it, the better. Start by just introducing him/her to the brush. Use the back side of it, let her sniff it, rub it on her paw for a second (the back side) give her a treat for accepting it. They aren't going to sit for a long time, once you get him/her used to it, then brush for a minute or less than give him or her a break. Do short sessions each day, (he head, the back, one leg, etc) even if they don't need to be brushed, and before you know it, you'll be doing her whole body and both he/she and you will be used to it and it will be a breeze to do!
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