Just blanket isn't enough, you need to clean all cum stains and put on all clothes she had when you started the night and blanket ofc
Also when the glasses guy comes to check up and Yuna is naked, cover her with blanket then hide, otherwise it also counts towards added suspicion
All in all, if you do allat you will still get some suspicion points (around 5 iirc) but it's very low and manageable
The above is a solution on how not to gain suspicion (or make it as minimal as possible), but to reduce sus value, you need to either skip nights, wait for rewind or if you have an item called literally "gift" - give it to her