I can understand why people are upset by this turn of events, but I think it's fairly plausible given how the characters were set up (aside from the breakneck pacing, but this manga has had that from the beginning). Makoto using Ritsu as a replacement for her sister makes sense -- she was introduced mistaking one for the other, after all. As for Ritsu, she's a people pleaser by nature and is absolutely terrified by conflict; recall how she cut off her hair and was prepared to leave Kyouko entirely after confessing to her. Of course her response to the woman she's trying to become friends with to get a lead on her sister is going to be to (attempt to) defuse and downplay the situation.
In a lot of ways, this manga is a (much) more melodramatic version of My Cute Little Kitten. Kyouko, like Yuna, has incredibly low self-esteem and subconsciously believes someone could only like her because they want to fuck her; Ritsu, like Rena, does not really understand her girlfriend and wrongly assumes that verbal assurances should be enough to set her worries at ease. (Of course, Kyouko's irrational jealousy of her girlfriend's female friend ended up being accurate, unlike Yuna's, but that's just coincidence; despite the saying, it remains paranoia even if they really are out to get you.)
I highly doubt this manga will turn into true NTR, but I do wonder where Makoto's character goes from here. It's going to take a lot of effort for the author to make her likable after this, but she doesn't strike me as a pure antagonist either.