Usher's Ex-Wife to Aunt -- Say It To My Face ... How'd My Son Nearly Die On Your Watch?

Usher's ex-wife has one serious question before her emergency custody hearing today -- how the hell is it possible to let a 5-year-old nearly drown if you're supposed to be babysitting him? -- and only Usher's aunt can answer it ... but she's nowhere to be found.

Sources close to the situation tell TMZ, Tameka Raymond attempted to serve Usher's aunt Rena with a subpoena to testify at the custody hearing today -- where Tameka will argue she deserves primary custody of her and Usher's two sons -- but Rena couldn't be tracked down.

We're told Tameka wanted Rena at the hearing so her lawyers could ask about what led up to the near-drowning of her 5-year-old son on Monday ... when Rena was supposed to be watching him ... but without the subpoena, Rena doesn't have to show up.

Even without Rena, however, Tameka has plenty of ammo to fight for custody -- and we're told her biggest weapon is that there was no drain cover on the pool the day Usher Raymond V nearly drowned.

As we reported, the 5-year-old got his arm caught in the drain and nearby contractors had to dive in to save him. Rena called 911.

Usher currently has primary custody of the ex-couple's two sons ... but Tameka hopes to change that today.