Can my sister get pregnant ?

My sister and her boyfriend have been trying to have a baby the last few years her boyfriend is way older than her he's old enough to be our grandpa he is a famous rock star though she's still trying to have a kid with him he's 69 he's gonna be 70 shes 28 this year he's got 3 kids from his first marriage she's never met them she doesn't want those kids are about her age maybe older. Every time she's tried to get pregnant she either has a failed test or she's had a miscarriage. She doesn't get why he won't be able to get her pregnant if ge ever does I'll feel sorry for that kid. I don't know why she's even trying for a kid with him. She's also been on birth control for about 5 years probably another reason she can't have a baby she'll be getting close to having it removed here soon.The lead singer for his band has a 2 year old daughter with his 30 year old wife he just turned 62.Is it possible he can still get her pregnant ? Why hasn't it happened for her ?