Disbelief is not inherently sinful. PLEASE disregard other posts by people who frankly don’t have the first idea what they’re talking about, and are handing out false and destructive advice.
Would your sister PREFER to believe, but cannot bring herself to believe? That’s not a sin. Sin requires an freewill act. We cannot “will” ourselves to believe anything. We can refuse to believe (which could be sinful), but we cannot compel ourselves to believe anything. I don’t sense that your sister is obstinately refusing to believe anything, so she is not sinning - not even venially. Zero sin.
A person may remain a faithful Catholic and disbelieve (not just doubt, but disbelieve) even in God (which means disbelief in EVERYTHING that the Church teaches). Even Saints have had such struggles (Mother Theresa struggled with this, as did St. Theresa of Avila). It’s called a crisis of Faith, and it’s not sinful, provided:
*]The person realizes that the disbelief is a personal flaw that should be corrected
*]The person makes an ongoing, diligent and good-faith effort to correct his/her disbelief
*]The person does not teach the disbelief as an alternative to Catholic doctrine
It is absolutely NOT a sacrilege for her to receive Eucharist under such circumstances. In fact, it is a small act of Faith. If she truly rejected Eucharist, would she participate? I wouldn’t. In fact, by not receiving, her actions could be perceived as a violation of the third condition I mentioned. By refusing to receive, she could be teaching (by her actions) that Eucharist was false. Anyone who knew of her doubts would surely perceive her refusal to receive as an expression of those doubts - and, now, she’s “teaching” her doubts. If she truly meets the first condition, she MUST receive (assuming she is not guilty of some unrelated mortal sin). Well, maybe “must” is too strong a word - we are never required to receive at any particular Mass, but she ought to receive if she is eligible, regardless of her doubts.