Agree that I prefer the more traditional spelling of Liliana. Paired with Marilyn, it sounds a bit long. Curious as to how many syllables you have in your last name.
Is Marilyn an honor name? If so I think it's ok as the honor is always more important than flow imo. If not, it's a mouthful and I'd look for something simpler with Liliana. And definitely Liliana.
Liliana (definitely spell it this way) and Marilyn are both pretty names separately but a bit much when they're together. I like that Marilyn is in honor of DH's mom, personally I would either find a less frilly first name, like Grace Marilyn, or move Marilyn to the first name spot and find something else for the middle name. Marilyn and Liliana would make a nice sibset, maybe save Liliana for any future girls?
I agree with previous posts about spelling of Liliana. Marilyn is my grandmothers name and it is lovely. Together, the two names are indeed a mouthful but in reality how many times do you introduce yourself by your full name? Except on paper and in rare instances, one won't being pronouncing all names together, so go for it!
Liliana Marilyn Brinson is a good name it is a little bit of a mouth full to say, but how often will you be saying it? It looks good on an official document and sounds professional. If you love it, go for it.
Re: Girl- I think I love this