No, in your household, 5 months wouldn't be too late. I have seen some situations where kittens 6 months old had kittens, but since your baby is inside with all "fixed" kitties, she should be fine. On the other hand, it's probably not too early for her to get fixed either. The vet at our shelter is fixing kittens as early as 6-8 weeks!! But I do know that they are doing it earlier and earlier nowadays. I remember when everyone had to wait until the 6 month mark.
I would go with what you think is best. I would say, sure she'd be fine to wait until 5 months, but if you want to go with the exam first to see if it can be done now, that would be fine too

Give that cutie a kissie for me!
**hugs**