Ririko probably knew that Yakuza-san is the bio father, but she didn’t want Meguru to be left in that lifestyle, so she decided on her best options even though they’re not blood related to the child. She was pretty irresponsible with her own life as well as Meguru’s though, sounds like she never acted like a mom anyways. Sentarou and Touma just happen to balance each other’s flaws perfectly, and just happens that Sentarou actually loved Touma, so it worked out, otherwise this whole situation would have gone VERY badly for little Meguru.
Ririko probably knew that Yakuza-san is the bio father, but she didn’t want Meguru to be left in that lifestyle, so she decided on her best options even though they’re not blood related to the child. She was pretty irresponsible with her own life as well as Meguru’s though, sounds like she never acted like a mom anyways. Sentarou and Touma just happen to balance each other’s flaws perfectly, and just happens that Sentarou actually loved Touma, so it worked out, otherwise this whole situation would have gone VERY badly for little Meguru.