If someone is said to be a presence it sounds to me like an esoteric entity (think of a ghost or some kind of mystical power). In German - at least on my account - people would not say to be a presence in this context but rather say his/her presence. More generally speaking (not just about my feeling), saying someone is a presence focusses on this someone's appearance. Hence saying someone is a presence refers to his or her charisma.
Therefore it is hard to express the exact same thing in German. I would recommend:
[Seine/Ihre] [Anwesenheit/Gegenwart] in unserem Leben war [immer/stets] von Wärme und Herzlichkeit geprägt.
Or with a little stronger emphasis:
[Seine/Ihre] [Anwesenheit/Gegenwart] brachte [immer/stets] Wärme und Herzlichkeit in unser Leben.
But as I said, both are not quite the exact same thing compared to what your mother wrote. Hence my recommendations are more a matter of taste than proper translations. Both do not make a statement about [the deceased], but about his or her presence. The first one might sound a little formal (von Wärme geprägt instead of just warm, which in this case would not sound right). The second one means, that [the deceased] brought warmth in our lives.
My sincere condolences by the way.