Ex's niece is his new lover
There is no such thing as being "too civilized," but you and your ex might be playing a mind game with each other.
This family seems to be pushing the two of you together (the mother taking your young lover to Victoria's Secret to celebrate your relationship is icky).
You have the freedom to have a sexual relationship with anyone; what you mustn't do is involve the kids in what is potentially a very messy situation. When this relationship cools (and it will), you will face the prospect of alienating your kids' extended family.
Whatever you choose, do not cohabit. You should explore your ability to have a relationship without leaping (or being pressured) into a domestic commitment.
