MY GF WANTS ME TO CONTINUE SLEEPING WITH HER SISTER TO SHOW I LUV HER

This started about 3 months ago. One day after having s3x wit my GF Jane, her sister Sophia walked in on us,I was still hard and thick.Nothing happened at that moment but later on my GF told me that Sophia had come up to her and asked about how it felt to fuck me,

My GF thinking nothing of it, told her,then Sophia stated that she was looking for a change down there from her husband who is rather small.

Hmmmm...,Sophia thought fuckin someone close to the family would be easier than finding a stranger.

My GF at first didn't like the idea of me fuckin her sister until she was further convinced by her sister,My GF is a real pushover,she did all she could so that I can do it 4 her, That she doesn't want her sister 2 go outside.

A week after the walk-in, we arranged a date for me to go to Sophia's house to fcuk her.We went to her house because her husband knew about this and was fine with it, hmmm...

The hubby doesn't want to lose Sophia ( She has had 3 kids and just recently got a lipo, 34DD implants and enlargement in the buttocks) and that's why he didn't mind. So we went on with it and sure enough she enjoyed it and wanted more but we had only agreed on one time.

After much pressure,we agreed that I can fcuk Sophia while my GF is on her period and only during that time but, of course we find other times to meet up. I still love my GF and don't really have feelings for Sophia,I really wanna stop this cos it's now getting to a length, imagine fcukin two sisters wit their concept,& I wanna get married 2 MY GF before the year runs off, but MY GF said that I should also continue fuckin her sis too if I really love her(MY GF) not withstanding we are getting married..

What should I do cos I don't want this to continue after marriage?

Will it be cheating if I continue this after marriage?

Is it really an adultery?

If you were in my shoes, would you have agreed at the 1st place, or what would you have done?

Plz don't take it personal,I only need your advice..