There's a rule that you don't put an article in front of singular family members (ex: mia madre, tuo padre, nostra sorella, etc.) but you add the article when there's an adjective/modification (la tua sorella piccola) and "mamma" and "papa" are considered endearing terms for parents, so they act grammatically as if they're modified by an adjective :)
Good question. My understanding of it is that if it's a 'term of endearment such as 'mamma,' 'papa,' etc. then use of the article is appropriate in contrast to padre/madre/fratello as you know. It may well be that along with being appropriate, it's also optional -- depending perhaps on context, regional usage, familial custom, etc. Here's where a native opinion would be helpful.
pete...you don't except from context since Lei can be both 'she' and "you-formal" . Sometimes you'd see "Sua" capitalized if it meant "Your", but not always. Not to complicate the issue, but it could also mean : "You are like her/his mother" since out of context the "la sua" could be either of those plus Your-formal again.
Hello to you also. It's not a matter of pro or not. Duo is a computer generated site. It has been "fed" a few correct answers for each sentence. Occasionally, (and much less of late) you'll find something Duo has note prepared the computer for a particular answer. Yes, even something as fundamental as "mom" etc..
You can help by reporting such issues in the "Report a problem" section and eventually (don't hold your breath) it will get fixed.. Btw far from being pro Brit Duo uses Am Eng. but accepts all correct Brir. versions. Another , btw "mama" is not really very much used in the US...I say this as an elderly, Am English teacher who has lived in various parts of the US.
Here are is a link for some tips etc and in particular the essential Guidelines: