Receiving the Eucharist unworthily is a grave sin. Your sister and brother in law should not receive Communion.
You are doing the right thing.
BTW, you can always come back to the Catholic Church. Just sit down with the priest and get your questions answered.
Same for your sister.
Grave sin doesn’t exist. All sin is sin to God.
Why does it seem that there is always the underlying assumption that someone is not Catholic because they have unanswered questions that a priest could answer and if they were answered, they’d come back to the church?
Not a chance.
I don’t have any unanswered questions about it. I know all I need to know.
And it wasn’t because of unanswered questions that I left in the first place or the second place.