Mrs. Sinclair doesn't have many redeeming qualities. She's a straight-up witch (and madam) who likes nothing more than making Clarissa squirm. While we don't get too much character development, we do learn that she'll do anything she can to help Lovelace achieve his evilest goal: the ruin of Clarissa.
In fact, the old brothel-owner is a little bit of a mystery. Why is she so intent on messing with Clarissa? Unfortunately, the one scene that might give us some closure is conveniently missing. That would be the scene: we find out that Mrs. Sinclair drugged Clarissa right before Lovelace raped her.
In fact, it seems that Mrs. Sinclair insists on helping. Lovelace tells us that "the old beldam, throwing herself into a chair, fell a blubbering and exclaiming" after entering Clarissa's room (256.57). The next thing we hear is Lovelace's report of the rape to Belford. Did Mrs. Sinclair convince him to do it? We'll never know. But we do know that we don't shed a tear when she develops a lethal infection after breaking her leg.