maybe edward simply did not care who his brothers married. there was a huge age difference and i doubt they were ever very close. as for their mother, as you pointed out, the boys were 17 and 18 and, unlike girls, could have married at whatever age, without the danger of spinsterhood. they weren't in line for the throne, so they weren't required to have any heirs, why arrange a marriage?
Thomas of Brotherton was heir to the English throne from 1307 to 1312, and until 1316 when Ed II's second son John was born, only the future Ed III was ahead of him in the succession. Given how many children died back then, there was at least a remote chance that he could become king of England.
Anyway, that's missing the point. Of course they were 'required' to have heirs - all aristocratic men needed heirs to pass their estates on to. That was the whole basis of wealth and status in society.
As for the lack of 'danger of spinsterhood', most royal men got married very young. Ed II was betrothed to Isabella at 17 at the latest, and would have certainly got married younger if Isabella hadn't been so much younger than him. Ed I, Ed III and Richard II were all married at 15. Ed II's cousin Thomas of Lancaster was 15 or 16.
Why arrange a marriage? For the same reason as practically all royal marriages were arranged: for political alliances. That's the puzzling thing about the marriages of Ed II's brothers - why wasn't Ed bothered about making an alliance with, say, Aragon or Portugal or anyone else who could be a potential ally against France? Why didn't he marry them to English earls' daughters, to create allies among his hostile nobility? Whether he was close to his brothers or not is irrelevant. If Ed didn't care who his relatives married, he was one of the very few kings who didn't.
