In general I'd rather be direct & let someone know their comments aren't welcome, but these kinds of faux innocent digs are SO difficult to respond to without looking like you're overreacting.
And I think you can become almost sensitised to the bitchiness so there is a real danger of overreacting.
I know I've taken the bait at times & it's played into my MIL's hands, so I love some of these suggestions.
Have never tried the yes / no answers - that's top of my list now!
Also like "I'm sorry, could you repeat that?".
I've found that refusing to understand / requesting clarification can also leave the one making the digs with nowhere to go.
"I don't think I understand your point" or "And you're telling me this because...?" are good ones.
If my MIL has made some broad, vaguely accusatory point - like your MIL saying your sister was moody - I've sometimes asked her to be more specific (she never can). For instance, "I've no idea what you mean - you'd have to give me some examples [shrug]".
Have also found it effective to just make some bland remark like "I can see that's what you think"; or to echo what she's said, as in responding to "we're concerned about you both" with a smile & "Oh, that's nice, we're concerned about you too".
For trivial but irritating remarks I sometimes just say things like "That's interesting", "Right" or "I see" in a tone of utter blandness, & then just excuse myself / change the subject.
If an offensive opinion has been expressed,but I know there's no room for an honest discussion (& there isn't with my MIL), then I have been known to say - in a pleasant tone, while smiling - "Oh, I couldn't disagree more!" or "Oh, I don't see it like that at all!". Then I change the subject, without any follow-up.
If it gets beyond this level, though, into real interference / nastiness, I would be prepared to be much more upfront- eg. if there were repeated remarks about your finances, or spending "DH's money", I would say "Sorry, I'm not going to discuss our finances with you" or "I wasn't asking for financial advice, thanks" - politely but firmly.