I am in a committed relationship with my girlfriend

I am in a committed relationship with a lovely  woman and things were going well until her younger sister suffered a painful breakup. 

A few months ago she discovered her boyfriend had been repeatedly unfaithful with a colleague from work It saddened me greatly, as I truly enjoyed our time together as two couples sharing meals, laughter, and good company Since the breakup her sister has been staying over at our place very frequently her office is nearby and more importantly she has needed the comfort and support of family during this difficult time My girlfriend and I have welcom*d her wh0leh**rtedly even before the breakup the sister was always quite relax*d and open around me in the normal way. I had seen her in underw**r or a towel after b@thing and once at the beach last year she chang*d her bik*ni t*p op*nly and remained without it bri*fly while we convers*d However, recently this openne$$ has incr**sed significantly. She often walks around the house in thin shirt$  without w**ring a br@ undern*ath. Many times in the living room she removes her cloth*s down to her br@ and th*ng on sever*l evenings she has even gone completely tople$$ for a while taking her time before putting on her nightw*ar

She has also started joining us on the sofa while we watch television settling in close with my girlfriend seated between us.

Yesterday was what truly surprised me. I was sitting on the sofa occupied with my phone when she lay down on the other end and placed her feet directly on my lap just as my girlfriend does when she wants a f**t ma$$age. I looked at her in sh*ck but she smiled playfully and said she was tired and needed to str*tch there if I mind*d I could move elsewhere I responded lightly saying it was truly her home and I was only a gue$t. Then almost without thinking I began gently ma$$aging her f**t through her s*cks as I usually do for my girlfriend she made a s*ft sound of appreciation at that moment my girlfriend ent*r*d the room saw the scene smiled w*rmly and announced that dinn*r was ready we both st**d up as if everything were perfectly normal But in my heart I am wondering has this sister’s comfort not gone too far? It has entered unfamiliar territory I truly need wisdom.