As others have said, trying to warn someone about a partner doesn't usually work and may actually hasten a marriage.
However, it does sound as though your DS and DIL were pretty worried and the critical and bad-tempered bits do make you wonder about the possibility of control or psychological abuse. There may, of course, be nothing at all to worry about.
Would you have any reason to give your niece a call or visit her and chat about things in general including her engagement? If so, she might say something that you could use as the basis for talking to your SIL without your DS and DIL being "implicated".
Maybe longer term , you and your DS and DIL could try your best to stay in touch with your niece, so that you can offer support should it be needed.
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