I have a friend who is in a similar situation. Her first child is 21 months while her younger one is 5 months old. This is what she has been doing: she puts her elder daughter in daycare and spends the the morning with her younger daughter. In the evening, when her husband is back, she will bring her elder daughter out for a walk (to the park or playground or the nearby market) to spend some time with her alone. That being said, she always feels guilty no matter who she is spending time with. I think there is no escaping the mummy’s guilt. But as long as you are trying your best to spend time with both children individually, and explain to the older one that the younger one will require more time (and could also ask the older one to also help look after the younger one), it will be alright. So far, my friend’s arrangement seemed to be working out well. Her older daughter does sometimes gets jealous and always try to make their evening walks longer. But she has not shown any animosity towards her sister (which is my friend’s greatest fear). Oh! My friend also showers both children with loads of cuddles and kisses whenever possible (especially to the elder one before bedtime, which is another of their “alone sessions”). Her main tip is to just try your best and not let the mummy guilt get to you.
I knew exactlty how you felt. My younger is now 11mo and my elder is 3.5mo. I always explain that my younger need more attention now as he doesnt know how to do things and he is young. Always get your elder involved when comes to taking care of ur younger child. Let them feel that they are EQUALLY important too. Mommy have to shown equal love too. Silbings are usually quite sensitive to favourtism. So bear in mind mommy, try not to have any favourtism!