If you meet a couple–a man and a woman–don’t ever ask, “Is this your mother?” even if she looks many years older than the man. We know a lot of horror stories caused by this faus pas.
Similarly, if you meet an older man with a much younger woman, don’t ask the man if she is his daughter. He may be flattered, but chances are he will not be. He may already be sensitive about the topic, and will resent being told that he looks much older than his wife, girlfriend, etc.
If you meet two women, and you suspect or know that it’s a mother and a daughter, it’s a risky compliment to ask, “Are you sisters?” Your question may be flattering to the mother, but not to the daughter who may resent being told that she looks old enough to be her mother’s sister.