All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
There is no doubt that pleasing the mother and giving charity are of the greatest acts of worship by which one gets closer to Allaah, especially when charity is given to orphans. So if you can convince your mother to allow you to continue donating for the orphans, then this is good and this is obtaining two benefits at the same time. But if you could not convince her and she insists that you give up donating for them, we do not see that you should stop this charity. However, you should do that without saddening her and do your best to conceal it from her, and avoid upsetting her even by using Tawriyah (saying something which has more than one meaning and intending a meaning different from what the listener is likely to understand). Imaam Maalik was asked by a man: 'I have a mother, a sister and a wife. Every time she sees something in my possession, she tells me to give it to my sister and if I do not do so, she insults me and supplicates against me." He replied: "What I see is that you should not irritate her and avoid her anger however you are able to'.
Lastly, it should be noted that paying back debts come in priority over optional charitable acts.
Allaah Knows best.