I haven't seen that movie so can't talk about it.
Abhimanyu and Sweety probably were getting somewhere before all this happened but they haven't reached the stage where they saw each other as life partners.
Sweety has no where to go now.She doesn't want to go back to Haweli and has no means to support herself so she may agree to a marriage of convenience with Abhimanyu.
Abhimanyu is lonely and was consciously or unconsciously looking for a partner.
If the marriage happens it is a compromise marriage for both of them.He will get a companion whose tastes are similar to his own and his daughter will get a mother figure.Alina stirred maternal instincts in Sweety and in that father daughter duo she probably saw a family that she could never have with Rituraj.
Ultimately it is not a bad deal for both.
Nimki right now wishes to fight her own battles on her own.I think she should be left alone to do that