Am I obligated to send Ramadan breakfast (Iftar) to my married sisters who are financially capable, and is this considered an innovation (bid'ah) if I am unable, and is taking a loan for this purpose, in order to please my mother, impermissible (haram)?

It is not an innovation to give food items as gifts to relatives or as charity to them in Ramadan or at other times. This is a legitimate act of kindness. You are not obligated to do this, whether you are poor or rich. If your mother commands you to do so while you are poor, you are not legally required to take on debt to obey her. However, if you do take on debt, there is no sin upon you.