@Springforward19 I think you are mistaken about what that Rhyme means, or rather what it doesn't mean.
It doesn't mean that daughters continue to hold their mothers in their hearts above everyone else. It just means that most daughters who have had loving and caring mums, don't suddenly start treating their mums like something the cat brought in, whereas many (but not necessarily most - I don't know of any statistics that cover that complex subject) DiL's do sadly appear to treat their MiL's as if their MiL's sole purpose in life is to make their DiL's lives shit.
Maybe many SiLs don't distrust or hate their parents-in-Laws, and maybe that is because most men don't find their MiLs, or indeed their FiL's, a threat like many DiL's seem to. I have been a DiL twice over nearly 50 years, and an avid eclectic reader of both fiction and none fiction, and unfortunately, Spring that silly little rhyme does seem to have some truth to it. My most dear and sadly long departed Mum, never changed from being the best Mum ever, but of course when I grew up and had children I wanted to live and spend the rest of my life with my husband, which didn't stop me from still holding my Mum in the highest esteem.
Our mothers were usually both Daughters and DiLs as well, as were their mothers etc.
So that rhyme, which is probably quite old, may show us that Son-in-Laws and Daughter-in-Laws have often had quite different opinions of their parents-in-Law, for quite a long period of time. Many old nursery rhyimes tell us quite a lot about the history of the time they first arose...