Our baby doesn't like to do anything?

she won't play with her toys or her 2 year old sister. when other company comes over (cousins, aunts, uncles, grandparents) she won't make any effort to be with them. she's 6 months and still doesn't make any sounds other than all the tears. she likes to be left alone and just sleep. all she does is sleep. you can't sing to her or hold her or rock her to sleep wiothoput her being fgussy and just wanting to sleep or nap. never giggles or smiles. it's breaking my heart. what shoul;d i do?