It's an unwritten rule to not invade others privacy I'd say.

I say that too.

But there are unwritten social rules, like "don't invade someone's privacy" - then there are specific, stated personal rules, like "do not do X with my baby."

By specifically stating a rule (even if it's also an unwritten rule - like "don't go through my stuff"), you're communicating that:

• you have a clear and defined boundary, and
• you expect the other person to respect that boundary.

A violation of a specific, stated, personal rule is usually a significantly greater breach of trust than a violation of unwritten social

This is because unwritten social rules run into all types of gray areas and can be interpreted differently based on different values/perceptions, without any malicious or dishonest intent.

Based on OP, MIL broke an unwritten social rule (that tells us that it's not okay to surprise-clean someone's house when feeding the cat). But she didn't break a specific, stated rule (e.g., "please feed the cat and do not do anything else in the house").

And OP only contains ONE breach of an unwritten social rule - there is no evidence of other breaches in OP. If DIL would really consider impairing her child's relationship with MIL on the basis of a single breach of an unwritten social rule, then DIL is probably the asshole.

But more to the point, there simply isn't enough info in OP to warrant an assumption either way.