Seeing as females can go into heat as young as 12 weeks now is when she could be able to breed (breeders tend to ween males at 8 weeks to avoid issues). Obviously she is too young so shouldn't be allowed to breed and pregnancy could kill her, but it's physically possible, meaning it's possible she has already bred with her brother (so keep an eye out for that in 111 days). I agree with not neutering, it's a major surgery, the testicles of a chin are pretty much internal, so they need to cut the chin open to get to them (like they have to do if a testicle doesn't descend on a cat or dog). Also as already said going under anesthesia is dangerous, but even if the chin survives the surgery, recover can be difficult. Chins in pain are known to not eat, so you will have to be prepared to hand feed the chin every few hours 24 hours a day until it eats on it's own. Also there is no health benefit for neutering chins, unlike cats and dogs, so it's a needless surgery if the chin is healthy, for the hope they will still get along after the recovery. I'm also pretty sure it's at least 8 week (2 month) before all the sperm is out of the system, so recovery time may be one month for healing up, but 2 months before reintroducing can happen.
I'm sorry if you already know this, but some people seem to think neutering is beneficial and a routine surgery even on chins, which it's not.
