I do not really agree on that realistic path. I know we prefer the fantasy that we seduce the virginal, perfect mom and sis that would never think of such debauchery and enter only into such a relation due to their love for us
Truth is though that it is always circumstances and/or an emotional need on both sides that lead to incest. Now MC might need to mature a bit and have to overcome some societal taboo in himself and Mom and Sis, but he already has the relation, love and trust with them he has build over his life with them to build on.
Also a fun fact for those that think MC would need to alpha up, in reality in almost all Mom-Son incest cases it is the mother that starts it and is the one in control actually.