Me gt the same problem with you. my #1 is 5yrs this year and #2 is 4mth.
before gv birth my #1 is very guai n can listen to what i say. but after birth she become 100% change. i feel so heartpain to keep scolding her and sometime have to beat her.
BUT she love her sister n want to get involve like feeding her meimei, see meimei bath n change diaper. so i change to soft approach n now she better than first few mths. she wil comment y we all love meimei n dont love her. so i slower talk to her n explain.
Many ppl tell me need take time one. Jia you! hope your #1 will understand you.