I will ask you to 1st consider this: how will it affect your relationship with your aunt?
I assume that you and your aunt are having sex for the pure pleasure of sexual activity, rather than because you are in some kind of romantic loving relationship. If you are romantically inclined towards her, then seeing her having sex with other men could trigger intense feelings of jealousy, which would definitely affect your relationship with her.
However, if this relationship is one based on mutually beneficial and enjoyable sex, and both you and she are excited by the idea of her having sex with multiple partners, altogether, then there is no really compelling reason not to do so. You do have to make sure that those involved are in good sexual health, and if you cannot ascertain this, then make sure they are wearing condoms. It should go without saying that your aunt should be using birth control, unless she is intentionally trying to get pregnant.
At 42 years of age, your aunt is still in the prime of her sexuality, and having come to enjoy having a young mother, she now wants more. In reality, there is no limit to the number of young lovers she could enjoy, except to the limit of their own comfort. But again, you must make sure that they are all in good sexual health, and if you cannot guarantee this, and they must be wearing condoms. Remember, if only one person in a group situation has some form of sexually transmitted infection, then he or she will spread it to all the other members of the group, and if they engage in sex with anybody else, that same infection spreads further, so making sure everybody is sexually healthy is of paramount importance.
So if you and your aunt have no seriously romantic attachment to each other, then you can enjoy this activity if you so desire, as frequently as circumstances allow.