Try to involve your daughter more with feeding and changing you son. When your son has a dirty nappy let your daughter help you change him, say things to her like how grown up she is because she uses the toilet all by her self, isnt she glad that she does not get all dirty when she needs to go the toilet, Take her to town on her own (if you can) and let her buy some really pretty panties. Buy her some special tissues and some special soap just for her.
Let her feed her brother, I know this will be messy and may take longer but she will soon become bored and want to do something else. Get her to show her brother what a good sister she is by showing him how to eat on her own. By letting your daughter become more involved with her brother she will not feel so left out.
When your son goes for his nap try to have as much one to one with her as you can. The housework can always wait till later.
Hope this helps, Good Luck