Is it physically okay for a 13 year old to have sex?

Well I lot a people lost there v plates around that age, but tell her be carefull about getting pregnant and catching stis and to trust the person cause she could get raped.

No. It is appropriate till the point that she is asking just for curiosity as kids at this age start exploring sexuality. So, it is perfectly normal to inquire. But indulging in sex before 16 age is not at all good and can leave deep scars. I have some friends who narrate there personal experience of repenting over this physical temptation. So, piece of advice. Make her aware of this.

I personally Don't think is good for a 13 year old to have sex. I don't even think their bodies are even mature enough to do anything. even if it looks like it.
Maybe she is being pressured into it, and being her big brother you have to protect her from that.

Physically, if she has had her period, her body is preparing and ready to have kids. But seriously, it really isn't okay, socially and mentally. Her body is ready for it, but is she?
I think you should tell your parents, or a trusted adult that wouldn't freak out.

Its not a good idea. I started at 12 and I regret it.