This looked like a tricky search as I don't have worldwide access on Ancestry, just UK. But a search for Leonie Casimir/Degueldre brings up a marriage record for Leonie Casimir and Jean-Baptiste Degueldre in Namur, and also a 1900 US census record for Leonie and Louis Staquet. I assume these are the records you're referring to, but I can't actually read them.
Switching to FindMyPast, though, brings up five entries in the Pennsylvania Passenger Lists record set, from Dec 30 1889, for Leonie Degueldre (27, Wife), Alice (7), Augusta (3), Eusebe (5 mo) and Lucien (5) Degueldre. They all have a birthplace of France (not Belgium, which may be an issue) and boarded the ship "Switzerland" in Antwerp, coming to the USA as settlers.
If this is right, it suggests that Leonie was still called Degueldre when she came to the USA, and so she may have married Louis Staquet after arriving in Pennsylvania. That should at least narrow down the search scope. FMP has the census record too, and the elder three children (Alice, Lucien, Augusta) match, while Louise was born in PA in 1891 after they arrived. So the marriage may have taken place before Louise's birth in May 1891. And if it occurred before her conception, then it's likely between Jan and Aug 1890. That's quite narrow (I wonder if Leonie travelled to PA to join Louis there and marry). I haven't found any passenger record for Louis Staquet, so it's not clear if he was already there or travelled around the same time.
Jean Baptiste Degueldre was not with Leonie and the four children on this voyage - this was a party of five. This is consistent with Jean's early death, but I don't know if Leonie would still be "Wife" rather than "Widow" on the form in such a case.
Leonie's card looks like this (I can put up the others if you want them):
The link, if you have an account with FindMyPast, is here.
So not an exact answer, I'm afraid, but I hope this helps narrow down the search.