Your sister has a point, but she's not 100% right about Jalen.I am due with a boy in December. My husband and I decided to name him Jalen after my husband's favorite basketball player, Jalen Rose. I shared the name with my sister and she gave me a weird look and said that Jalen is a girl's name. She said we should name him Jayden so everyone will know that he's a boy. Is she right?
When Jalen Rose started playing for Michigan in 1991, his name jumped into the national spotlight. It debuted on the top-1,000 charts in 1992 at an impressive #377. It trended generally upwards, peaking at #106 in 2000, and declining since then. It's still a relatively popular name for boys at #393. The alternative spelling, Jaylen, is even more popular, at #203. So, if you name your son Jalen, he will certainly not be the only one around. Jalen has never been in the top 1,000 for girls (though Jaylen is #906 for girls).
The problem is that names ending with -ayla and -aylin are very, very popular for girls at the moment. Your sister probably knows several little girls named Jayla (#132), Kayla (#49), Kaylin (#399), Jayleen (#452), Jaylee (#612) etc., not to mention sound-alikes like Kaitlyn. When your sister says that Jalen is a girls' name, she probably doesn't mean that she knows a bunch of girls with that name, but that the -aylin sound is common in feminine names.
It's true that there are lots of little boys named Jayden — in fact, if you aggregate all of the spellings, it is the 2nd most popular boys' name in America (after Aiden). The thing is, it's also fairly popular for girls (#212). If it's important to you that your child's name not be gender-ambiguous, changing from Jalen to Jayden won't help.