Quoting vivalacandy:" continuing joint custody (he was going to get her every friday-sunday evening) i dont think the letter ... [snip!] ... legally...my main concern is my daughter feeling abandoned by me or passed over while her baby sis is with me full time..."
So, then you need to take it back to court.
She won't feel abandoned or passed over for her sister as long as you don't project that feeling on her.
Also, you better hope that letter DOES have legal weight because if not, you just broke your court order and you could be found in contempt.