The ONLY name my husband and I have been able to agree on is [name_m]Marcus[/name_m]. Does it work as a sibset with [name_f]Greta[/name_f] and [name_f]Giselle[/name_f]?
I think it sounds just fine 
I think it sounds good as well!!
If might be more “flowy” to have a boys name with a ‘g’ somewhere in it - preferably not at the start. Perhaps [name_m]Magnus[/name_m]? Bonus - it’s very similar to [name_m]Marcus[/name_m]!
Other sib set considerations: [name_m]Angus[/name_m], [name_m]Fergus[/name_m], [name_m]Hugo[/name_m], [name_u]August[/name_u], [name_m]Gideon[/name_m], [name_m]Garner[/name_m]
However, [name_m]Marcus[/name_m] is classic like your daughters’ names and nice masculine balance to their femininity! If you both love it then it’s the best!
A very handsome choice! I think it works very well.