When expressing sympathy on a card, one phrase that fits well is, "Please accept our most heartfelt sympathies for your loss. Our thoughts are with you and your family during this difficult time." Because sympathy entails sharing feelings with someone else, especially in times of sorrow or trouble, it is important to show feeling and compassion when writing the message.
Another option for a sympathy card message is, "We are sorry to hear about the passing of your sister. We hope that you are able to find strength and comfort in your time of need. We hope it helps you to know the lives your sister touched, and that her spirit will always live on. She will never be forgotten."
One other message that fits in a sympathy card is, "We were sad to hear of the passing of your mother. We are here to support you in any way possible. Please know we are thinking of you."
If only a quick message is needed, something like this fits, "We are remembering you and your father in our thoughts and prayers as you go through this difficult time.e."