I don’t think making one an ella and one elle actually makes them work together any better; I actually think it makes it even more of a tongue twister - sorry
Between [name]Mirabelle[/name] and [name]Mirabella[/name], I like [name]Mirabelle[/name] best. I think the name is frilly enough without the extra a.
Between [name]Giselle[/name] and [name]Gisela[/name], however, I love [name]Gisela[/name] - I think the a softens it and adds to the exotic feel of the name.
In terms of a sibset, I would keep [name]Gisela[/name] and look for another name to replace [name]Mirabelle[/name]. Actually, I think [name]Gisela[/name] and [name]Mira[/name] are wonderful names for sisters. If [name]Mira[/name] feels a bit insubstantial to you, it could be also a nn for [name]Miranda[/name]. [name]Miranda[/name] and [name]Gisela[/name] are perfect together!
I have the same issue…my daughter’s name is [name]Gabriella[/name] and I really like the name [name]Isabel[/name], but I think those two sound way more similar than [name]Gisela[/name] and [name]Mirabelle[/name] (not sure why!). If that makes you feel any better.
Of the two combinations, [name]Mirabella[/name] and [name]Giselle[/name] feel more natural to me, somehow, even though it makes them an unequal number of syllables.
Unfortunately, I do think they are too similar. I would pick [name]Mirabella[/name] for the lovely nn [name]Mira[/name], but [name]Giselle[/name] is gorgeous too (and can give the sweet nn [name]Gigi[/name]). Whichever didn’t get used in first place would make a stunning middle for another daughter.
No, I would not ever consider using [name]Mirabelle[/name]/[name]Mirabella[/name] and [name]Giselle[/name]/[name]Gisela[/name] together, due to that common “elle” element. Adding or subtracting an A doesn’t make a bit of difference in my mind. Sorry!
They are both lovely names, though! I prefer [name]Mirabelle[/name] and [name]Gisela[/name], myself. [name]Mirabelle[/name] sounds more whimsical, playful, young, and pretty whereas [name]Gisela[/name] sounds exotic, sophisticated, elegant, and strong - as I said, I really adore both! Good luck!