Dear Queenie,
My sister is very smart and well-educated, but she got engaged to a man who is not as well-educated as she is and it seems to me he is actually rather stupid.
Queenie, I really think he is not good enough for her but she has never asked me what I think about him.—Big sister
Dear Sister,
If your sister has never asked you what you think about her fiancé she obviously is not interested in your opinion of him.
It may be hard to understand what she sees in him that makes her want to marry him, but asking her about it and/or telling her what you think could cause problems between the two of you because she probably would be vexed that it is not complimentary and would not pay much attention to what you say.