M.F.
My friend has the same problem. Her daughter is 3 1/2 and younger brother is amost 11 months. She got her daughter little toys that the baby cannot play with and in order to play with them she has to go into the playroom and mommy stays with the baby in the living room... She has taught her that if she does not respect her brother and let him play also when the big toys are out then she gets ignored no matter how big the tantrum, she is old enough to leave in a room alone. She also makes sure that the older one gets one on one time without the baby at least once a week and once a month they go on a mommy daughter lunch/movie/park trip and then when she gets her special time with daddy then mommy ends up getting baby time also!! I hope this helps you out and wish I had more for you....