Ah. Then it is as others have said, she needs to be (gently and lovingly) reminded of church teachings shown in the Catechism. No one should receive Holy Communion while in a state of mortal sin. Unfortunately, for many the fact that divorce is legal, and that one may thus legally remarry after divorce, may make them think that the remarriage after divorce is not sinful–how can it be sinful if it is legal? (See: abortion, homosexuality, fornication, et. al.)
In this culture where everybody feels ‘entitled’ to whatever they want, whenever they want it, and where personal sin is regarded as practically non existant (how am I ‘sinning’ if I am legally divorced and legally remarried? I’m in a ‘legal’ marriage! I’m not harming anyone! The church needs to get with the program here! I’m not a sinner!), it may be very difficult to get a ‘nice’ person convinced that some action of hers may be, not just ‘sinful’ like a white lie or not recycling etc., but mortally sinful. When many persons tiptoe around not daring to consider anything other than God’s mercy and how all He wants is for us to be happy (this is a very distorted picture of God which many Protestants and not a few Catholics have become convinced of), even hinting that there are things people do which separate them from that benevolent and politically correct entity will raise hackles. You have my prayers as does your sister. God certainly does want her to be happy, I’m not saying He doesn’t, but He does not want her ‘only’ to be ‘happy’ according to desire, which is transient and earthly alone, but to be happy in following Him and His will. . .which has little to do with one’s personal ‘satisfaction’ and everything to do with humility, self-sacrifice, obedience even if causing personal distress, and recognition of an authority that supersedes “me”.