S.H.
ETA: okay, so let your Husband/the friend, deal with your older boys. And YOU... handle the infant.
And, you ALL talk about this, BEFORE you go on your trip. SO that, Husband is full aware... of his plane/trip duties with the kids. Otherwise, you will likely be the one doing everything. And make sure, Husband knows he has to help. Not just socialize with friend and/or just sleep on the plane ride leaving you with 2 full-awake active fussy 4 year olds.
----------------------------
I hope, you will ALSO have some other adult... going with you on the plane and on that trip.
Your arms will be full carrying stuff. And with a baby and baby stuff and nursing baby etc.
My friend, has a daughter with twins. And an older child. SHE... is flying to where her daughter is, and then, accompanying her daughter and her THREE kids... to their trip's destination. Then, it will be a vacation for all of them. Then she will fly back with them all, and then go back home herself.
There is no way, her daughter can, by herself, do all of that.