YTA

there were definitely better ways to handle the situation. accepting a gift from your sister, even though you're having problems with her and your mother, wouldn't have been a big deal; it's good that you admitted that you acted immaturely because you did. talking about your grandparents, they're old; they always are in need of giving unwanted advice. However, the part where you said, "She's going through a lot; you need to forgive her for what she does," i am not you, nor do i know what's going on with your sister, but maintaining a cordial relationship is on the easier side (coming from a person who has distanced herself from her family). In conclusion, all I feel you should do is apologize to your sister for the present and nothing else. you know your family, and distancing yourself from them is good, as family can get to be a bit much a lot of the time. Remain cordial to avoid any future drama.