i have learned with my son if i put a diaper or pullups on him he will go in them. when he has his "big boy underwear" on he uses his potty chair everytime. i also put his potty chair out in the hall where i can monitor him better because when he is in the bathroom he lkikes to play in the water instead of goin potty. try it. it  worked for me

My niece had this same problem when her little sister was born. Her mom included all of us (aunts uncles grandparents) in showing her how cool is was to be a "big kid". We had her help us with things like cooking and cleaning and helping with the new baby while telling her that since she was a "big girl" she could help us with those things. We also reinforced that "big girls" used the potty by showing her how tiny the new baby's diapers were and all the pretty big girl underwear she could have. The biggest thing that helped was letting her have some decision making involved. We had her pick where her potty went and what underwear she was going to wear that day. Within two weeks, she was calling me saying "Auntie, I'm a big girl. I don't need BABY diapers. I'm using the potty." It's been two months and now she is even potty trained through the night.

The same basic thing worked for my son (only there was no second baby for him to contend with).

Our two and a half year old has shown us that she understands and can use the toilet but sees her 1 yr. old sister getting changed and chooses not to use it. Our saving grace is she loves older kids and will be hanging with them all summer at the pool. We have simlar problem with the bottle.

My son started showing interest in the potty at about 27 months.  When he started showing interest we did reward RIGHT away.  He got a sticker chart at day care and m and m's at home.  With both rewards he gets 1 for pee and 2 for poo.   He trained REALLY fast and rarely wets anymore.  Also, during the first few weeks if he was dry at day care all week (excluding nap) he was able to pick out a package of big boy underwear at the store.  That also worked well!  Good luck!