as long as it's a mary sue for comedic purposes, or for relaxation, it's fine. as soon as it drifts to bullshit wish fulfilment, then that's where the shit starts. (redundancy).
The baby wizard could become a part of MC's alpha team. I mean why should it be only consisted of harem women? Ofc the baby could become jealous, but then again the baby got the 3rd sister as a babysitter.
The eye is probably like Sharingan - the more you use it the more it hurts until the eye turns blind. (not sure about blind in this manga's case)